Lightroom not correctly searching by keyword
I have a strange problem that I can't figure out on Lightroom 3 regarding keywords. I try to keyword important photos by entering information in the keyword field. Thereafter, I can go to the Library, hit the Text button and search under keywords. For almost all the keywords I use, it works fine. E.g., I have a keyword with my daughter's name and it only shows photos with that word in the keyword field. I have other keywords based on subject (e.g., flowers, birthday, etc.) and they work fine.
I just got a new dog named Ace. Naturally, I tagged all of his photos with the word "ace". But for this keyword (and this keyword alone), it shows photos that don't include the word or letter combination "ace" at all. For example, it shows a photo with two keywords, "home, Chandler". It'll show other photos that only have my daughter's name. The word or letter combination "ace" doesn't appear at all in the keyword field or anywhere else in the metadata panel (which shouldn't matter because it should only be searching the keyword field anyway.)
Changing the search from "contain" ace to "contains words" ace solves the problem. But I would like to figure out why it's showing other photos instead of just the photos of my dog.