- You have a cored deck
- Your hole happens to be located between the webs formed by resin filling the scores in the core material
- Your hole is not located where plywood reinforcement was used - where most load-bearing fastenings will be made
More over, there are several rainy afternoon's-worth of excellent reading there on the right/best/easiest way to do things. (When pulling a cutlass bearing, does it really matter where you make the saw kerf? Yes!)
Check it out - you won't be disappointed.
* Thanks to Erik at Erick's Wanderlust Blog for finding this great site, and to Scott at Downeaster Yachts for pointing it out to me.