need a soft doubling explanation

I play a practice game w/ my girlfriend as the dealer. When i'm dealt a soft doubling hand (eg. A,7 vs. 4 or A,6 vs. 4), i proceed to follow basic strategy. She (not knowing any strategy) finally had enough and asked me "why would you double that when you already had 17 or 18". The only thing i can come up w/ is that the dealer is most likely to bust with a 4-6. Help!

bac