A question about reverse polarity

A friend here sent me a preset that includes two U87 figure 8 mics. The two mics are paired and at a 90 degree angle. One of the mics has its polarity reversed (red in the mic mixer). This is new to me.
My guess is that this configuration is a variation of mid-side technique which provides both a good amount of stereo detail with mono compatibility. Am I on the right track or is there something I don't understand here. Thanks for any thoughts you may have on this matter.