When your 13th lvl Seeker is able to routinely end freaking gorilla liches, some paltry 5 percent increase in accuracy you missed back on level 1 doesn't really bother anymore. Yeah, going from +4 1d10+4 to +5 d+6 is banging at first, it quickly loses its luster.
The smallest possible increase in to hit on a single attack is 5.6%, which represents hitting on a 2 instead of a 3. Even then, it would give a 7.7% increase on your second attack (hitting on a 7 instead of an 8), and a 12.5% increased number of hits on your third attack (hitting on a 12 instead of an 13).
In short, your level 13 character is getting at least 8.6% increased hits on a full attack.
My only assumption is that the +1 to hit increases the to hit probabilities on all attacks. I.e., no hitting on a 2 or only on a 20.