Few months ago we had some discussion on this matter. Another user said about the same thing. But we couldn't find the main reason behind it. The thing I noticed on that case
@NJ89, actually you received higher interest than you should. From the interest calculator I can see that it will give 9 satoshi for having the amount in account but you have received 10 satoshi in between.
The interstate amount changed in the same way in your case. Instead of getting 131 satoshi you have received 132 satoshi on 20th September. From the freebitco interest calculator

I think
‘Saint-loup’ reply was reasonable there.
Thanks for sharing this post. Yeah, Saint-loup mind that they rounds Satoshi to upper or lower number makes most sense and I also had such prediction in my mind. But still, it would be interesting to get input from TheQuin about it.