Small selection bug with Find and Replace

This one should be pretty straightforward. I am replacing a 3 letter word with a 4 letter word and after replacing several instances within a selected area of text, that selected area has shifted (gotten smaller) by the same number of characters. You'll want to test it a bit with different lengths to see exactly what it's doing...

This picture should show you exactly what I'm talking about. http://stevenlu.net/files/12_4_24_screenshot.png