Glad to see this post because this happens to me all the time....misery loves company I guess. Anyway, your post got me thinking this morning so I retreived my carpenters level and stopped by the garage to check; there is a substantial rake/slope to the windowsill. If you line tthe sill up with the parking spot divider line you'll be off parallel by the degree of rake. Actually, this is a relief to me because now I can go back to believing I'm not losing it.