I can't say with certainty the reasons why these decisions were made, because these decisions were made many years ago and I wasn't involved in the development of this feature in any way.

From my point of view, some of the design choices seem a bit strange. My best guess is that the developer(s) were concerned about computationally loading the network nodes.

Why is there a 30 days period? can it be short or longer or daily?

Yes, it could be made shorter or longer. But we haven't done any performance analysis of changing this aspect of the algorithm (although my gut feeling is that there would be little negative impact from reducing it some). Truthfully speaking, we didn't want to spend a lot of time on this feature, we're mainly focused on developments that we believe can have a higher long term impact and I didn't think changing the interest rate period would make much difference.