In recent days I noticed strange thing on my account. Sometimes I get less interest than on previous day, despite that my account ballance increased and I should get at least same or bigger interest than on previous day. I'm not complaining that I get 1 Satoshi less than I supposed to get, I'm just interested why such thing happens.
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.