Thanks Catymag and Tom1807! I'm using the patched 6.16.3 Mapsource.

I've tried setting the details to Highest and then I could see all details. After that, I could see the details even setting to High or Medium. I think the "problem" was caused by the Mapsource cache (C:\Documents and Settings\Y-O-U-R--U-S-E-R-N-A-M-E\Application Data\GARMIN\MapSource\TileCache). What do you think? Anyway, it's solved...