Why should I discard 7pin here and not 5pin?

This is the extended South round in an East match, because everyone was below 40000pts at the end of the last East round. Consequently, I only need 1100 points to win instantly. My hand already has 2 han, so I do not need this red 5pin. Therefore, I decided to cut it early for safety (sakigiri).

And yet the AI chose to cut the 7pin of all things! I mean sure, I planned to get rid of the 7pin right after anyway, but the discard order is crucial in mahjong, and the red 5 being a dora and more in the middle makes it more dangerous, so better for sakigiri against a potential riichi from kamicha (both 5pin and 7pin are genbutsu against toimen anyway). Also, if I happened to draw the last 8pin, I would unlock more options, so that is one more advantage of keeping the 7pin over the 5pin.

I believe Riichi City's AI is Mortal. Sadly, it does not show percentages, so we cannot tell how much it hesitated between its first two picks. But in this case, you can see on the right panel that the 5pin is not even the second pick, it is the third, so evidently the AI considers that discarding the 7pin is better than discarding 5pin by a significant margin. Which does not make sense to me. How can keeping a lone red 5 like this be even remotely useful?