I’m not sure I’d classify this as a bug, but it certainly can be confusing as the number of shown posts depends on both the setting value and an external factor (the page height).
I’m also not sure what would be a use case of the topics per period in top page setting, as the /top view could be considered a topic list like any other unless there are things I’m unaware of that make this setting relevant. 
I’ll move it to Contribute > Feature for now.