The quick answer: Your statistics for reply rates etc ONLY count the last 60 days.
If you missed 3 messages 59 days ago, then didn’t miss ANY, on day 61 your response rate will go up because those old missing responses aren’t counting anymore (even if you aren’t getting any new messages to respond to)
Similarly, if you responded to 10 messages every day for a week 59 days ago, then missed a message a week later and didn’t get any new messages, your response rate will go down every day because those old messages you did reply to are no longer being counted, so ever day that passes the percentage of messages you responded to in the last 60 days will be going down.
E.g: Lets pretend it’s 7 days instead of 60.
On Monday, Tuesday, Wednesday, Thursday and Friday you reply to 1 message each day. Your score is at 100%. On Saturday you miss 1 message. Your score is 83%. You don’t get any messages on Sunday.
On Monday, you reply to 1 message. But your score doesn’t go up! Why? Because last monday doesn’t count anymore, so you’re still on 83%. On Tuesday, you don’t get any messages. You haven’t missed any more, but now your score is even lower! It’s now 80%, because LAST tuesday doesn’t count anymore. On Wednesday, you don’t get any new messages, and your score is now only 75%, because last Wednesday doesn’t count anymore.
So basically, your score is lowering because your successful replies have past the 60 day mark and aren’t boosting your score anymore, so having missed ANY messages in that last 60 day period will make your score go down until the number of messages you ARE responding to matches the rate at which they’re elapsing