When it's counting down tenths of a mile, it makes the announcement for one-tenth of a mile, or 500 ft when the 0.2 turns to 0.1. This seems logical, but as Alan says, it's actually at .19 without that 9 visible. The announcement gets made, and then 10-15 seconds pass and then it starts counting down in feet...starting with 475 ft.
It should be switching from 0.1 to 500 ft (or 530 ft for that matter) and that's when the announcement should be made. Until I saw it happening, I couldn't make any sense of it.
Also, it seems to chopping off the "In" with TTS. Instead of "In 500 ft, turn" I'm only hearing "500 ft, turn". It doesn't seem to happen every time though. The two times I noticed it, I was turning right.