In custom partitioning, if you 'add a new mount point' without specifying a Desired Capacity, anaconda tries to figure one out for you. it seems to use the 'available space' counter for this function, unconditionally - it just makes it as big as you have 'available space' remaining.

This is as good a way to do it as any for most cases, but it has a weird result in one corner case. If the default type for new devices is set to LVM (set by the dropdown on the Installation Options dialog, LVM is the default), and you have an existing VG with free space inside it and very little actual unpartitioned space remaining, when you create a new mount point, anaconda will first try to create a new VG, and - when that fails - instead create the mount point as an LV within the existing VG. That's smart! However, if you don't specify a size, anaconda will use the 'available space' to decide how big the new LV should be, even though in this particular corner case, that doesn't make sense. So even though you have 50GB or whatever of free space in the VG, your new LV will be created as 900kB or whatever tiny amount of space you had available outside the VG. (If the size you wind up with is less than 1MB, this will actually trigger an orange error bar).

This is a much bigger problem in combination with https://bugzilla.redhat.com/show_bug.cgi?id=906906 - a related problem with the editing options which means you can't edit the size of the newly created LV. If 906906 gets fixed, this gets relegated to more 'annoyance that looks pretty kooky' than 'major problem'.