The system is looking at sensor #1 as X and #8 as Y. If X-Y = >13.25 Deg for more than 1 second with the airspeed >60kts, that is one of many things which will put the flap bypass valve into bypass.
The best answer I can get, that makes sense, is that the flap position sensors have an 800 to 1 mechanical turn ratio while the skew sensors are direct drive making the angular position difference easier to calculate in the FSEU.
(and yes, my oops for forgetting that only the outboard sensors are monitored for UCM)
From SDS 27-59-00-002:
"Physical Description Each flap skew sensor has one resolver that uses 28v ac 400 Hz excitation. The table below shows the resolver degrees when the TE flaps are at up and at 40. For sensors 1 thru 4, TE flap extension causes the resolver degrees to increase. For sensors 5 thru 8, TE flap extension causes the resolver degrees to decrease.
Flap Skew Sensor Outputs in Resolver Degrees
Sens #1 2 3 4 5 6 7 8
Up: 303 338 6 340 20 354 22 57
40: 233 288 297 274 86 63 72 127
An input rod connects to the flap carriage and the skew sensor input assembly."