Currently the Marketplace main page lists the new Add-ons in the first row below the buttons for Skin Packs, Worlds, Textures & More, and New. If I scroll the row to the right to the entry labeled "See All" and click that, it performs a search that returns 15 add-ons. But if I instead click the new Add-Ons button in the menu on the left side of the main page, it only shows 12 of those 15, selected at random.

I suspect that when you select the menu button, the logic does a search for add-ons and builds rows of titles, one row at a time, adding them to the result page. But if the last row is incomplete, it doesn't get added to the result page. If my conjecture is correct, the actual numbers will depend not only on how many results there are but also on screen geometry and graphics resolution, and if the number of titles happens to be a multiple of how many fit in a row, none will be missing from the output.

Note: It's impossible to write a list of steps that reproduce the problem because currently the only method of determining how many titles actually exist in the Add-ons category is to scroll the top row to the end and click See All. (I couldn't find a set of search keywords that only returns add-on titles.) But the top row isn't permanently dedicated to Add-ons and will eventually be reused for something else. When that happens, I'm not sure how (or even whether any way will exist) to demonstrate the problem.