Why is \topfraction larger than \bottomfraction?

One general remark on the here placemnt: A figure can only be next to a reference to it when there is only a single reference to the figure in the text. If there is a second, maybe just one sentence below, the figure might still be close. This works well for things like "...see the following example".

However, if you refer to a figure, or maybe to parts of a figure, more than once or twice, there is no proper "here". Thus, one puts the figure to the top (or bottom) of a page. Then, at least as you are on the same page, you can read the text without any breaks and easily find the figure the text refers to. This, I think, works for top and bottom placement.

However, if your text that discusses the figure spans more than one page you will have to turn pages. Of course, readers are used to start reading at the top of a page, this is why I think that top placement is preferable: After turning the page, you will first look at the top and you will find the figure you are looking for. Even if you turn the pages backwards.

Of course, this requires an easy-to-recognize optical difference between caption and text. And this is not true for all publishing formats. While it's fine for anything on paper, if I read a pdf on the screen, I have to scroll over the entire text of the previous page to find the figure. Here, bottom placement seems to be better.