I'll try to explain. Wizards, after releasing 4th edition, realized they screwed up with the math, because apparently basic addition is very complicated; characters' to hit bonus advanced slower than monster's defenses. Instead of errataing (sp?) the whole monster manual, which they sort of ended doing anyway with MM3, they released a feat called Expertise which everyone was supposed to take sooner or later to fix the math. The good judges of L4W, realizing that everyone would simply get the feat and thus there would be very little variation between characters (aggravated by the fact that most characters here are quite low level and have at most a couple feats) simply banned the Expertise feat and gave everyone a +1 bonus at level 5, +2 at 15 and +3 at 25 IIRC.
Later still Wizards realized they were screwing up the characters that used both weapons and implements, like paladins; they had to take two expertise feats to "fix" all their attacks. They therefore released a feat that was strictly better than Expertise, just to confuse everyone. They called it Versatile Expertise, which gives you the bonus with one implement and one weapon. This feat is, as far as I know, allowed in L4W and stacks with the free 5th level expertise bonus.