Well fuck I was looking at this for you and got tired of the "server busy" message.
So, I'll tell you this. Google doesn't always see dashes as literal hyphen characters. It sees it as spaces sometimes, so the 991 results are pulling in the TMA-<something> phrase because it's picking up TMA as a separate word from the appended data after the hyphen.
Google is contextual, so it will try to "guess" what you want by the content within, and it is very good at working through code errors or corrupted data. It's possible that the BL search is much more static, unforgiving of errors, and literal. Actually, I'm sure it is. Where Google will try to pick up "TMA" and its variants, vBulletin will only pick up TMA and nothing else, including if there is any data corruption or extra characters included. For instance, if there is a space in the title, it might miss it (guessing here).
I know it's hard for people to understand, but these little search engines are very specific where Google has an entire team of people who create synonyms to words so it can understand the context of the phrase you're typing in.
I was gonna try to look, but server busy is annoying me.