Why default presets have low notes detuned flat?

I wonder how much this question involves the difference between 6:3 octave tuning and 8:4 octave tuning in low notes.

6:3 tuning is where, when tuning two notes one octave apart (say C1 and C2) the sixth partial of the lower note (C1) is tuned to match the 3rd harmonic of the higher note (C2), and 8:4 is where the eighth harmonic of C1 is matched to the fourth harmonic of C2.  These tuning "baselines" (which also include 2:1 and 4:2 as common choices--with rare 12:6 situations as well) are ways to correct for inharmonicity in a piano across its full range, and generally one of these is selected according to the technician's preferences or specific experiences with a given instrument.  Sometimes there is even a shift between different octaves where some are 6:3 and others are 8:4.  Generally a technician uses a precise electric tuning aid set to 6:3 or 8:4 and then, nearly always, finishes the tuning by ear to suit what sounds best to them.

As the ratios get higher, the tuning feels generally "wider" but at the expense of technically being flat.  When done tastefully it shouldn't be unpleasant but it can have poorer results on some instruments--particularly smaller ones--or, theoretically, for some genres and styles.  While I haven't studied carefully if the Pianoteq presets are specifically 8:4 or not, I know large Steinways respond well in practice to 8:4 because of that wider sound it gives; therefore I'd wouldn't be at all surprised to find that PTQ, either by deliberate design or by emulating real-world Steinways, incorporated that physical aspect of widened tuning in some way or another in the modelling.

Using Standard--and even more so in Pro--you can bring those notes more into tune, if you wish, but it can increase the inharmonicity (usually mitigated by increasing string length) or change the balance of low bass and high treble (as treble notes offer dichord and trichord unisons that can be tuned wider to match the overtones of different octaves which the lowest notes cannot), so it can take a lot of work to resolve this note-by-note in an instrument much as a technician tunes the entire piano very slowly, but we do have tools in PTQ to do most of it with one click.

Low note tuning can vary substantially in how much "flatness" is acceptable.  As long as the tuning sounds consistent, it is considered accurate, but if the tuning sounds inconsistent, then there's a problem.  Clients might want a "tighter" sound or "wider" sound, and then it's best to fit the preference of the client.  If the flatness is a problem for your needs, then definitely change it using Qexl's solution to see if the sound improves for your needs, and if that improves but doesn't fully resolve it, then keep refining the options in the tuning and detuning window until it does.  I should note that Pianoteq's implementation of "Flat" temperament is Equal temperament with no octave stretching at all, and if there is octave stretching enabled in a preset, it will be disabled, so any stretch setting or note-by-note edit setting in Pro will be bypassed by the engine.

As a side note, American technicians tend to favor more octave stretching and narrow unisons and European technicians tend to favor very little octave stretching and much wider unisons.  Both approaches tend to reach the same result of blending the inharmonicity of an instrument into a pleasant (and logical) sound, but they are very different in how they're implemented and how the many differing, resultant overtones are managed (or run wild), especially when an instrument is played with a lot of pedal resonance.  While I prefer wider unisons with less octave stretching on my own instruments and recordings, I can't think of any performance or recording where I've felt that the piano's unison width or octave stretching distracted or detracted from a performance: by the time a performer is making real-time decisions about how they want to utilize the strengths of the instrument they're playing at that exact moment, there are so many more things that I start to listen to, that the scientific questions of the piano technician (at least whenever the technician has done their best work and there aren't any sourly out-of-tune notes or other mishaps) melt away, showcasing only the artistic questions of technique and interpretation.