Craft Trap <=> Engineering

[NothingGoldCanStay]

As I understood it, Engineering was supposed to work like Secondary X, but for Craft Trap, in that every 5 ranks should give you +1 to the modifier of Craft Trap. I checked today, and my level 7 wizard with 17 INT and maxed Craft Trap only has 13. (10 base + 3 INT) but no +2 from Craft Trap.

Is this not the way it's supposed to work, or is this in fact a bug?