Can anyone help me understand the matches to this regular expression?
Well let's walk through it. You need one or more vowels, followed by any number of numbers (including none), followed by (optionally) a letter.

ae matches your vowels, 12 matches the numbers, then a counts as your optional letter. The engine starts again and looks for the next vowel. It skips bc r2d2 since there are no vowels there.

Then it finds a for your vowels, 1 for your numbers, and s as the optional letter. Then the engine starts again and looks for the next vowel. 

It finds ea for your vowels. There are no numbers, so it grabs k as the optional letter. Then it starts again looking for vowels...