There are two trains of thought on this. Both assume that the 400 series steels are the same varieties we are all familiar with.
1) They are using the absolutle cheapest 400 steel they can find (420, 425, etc) and don't want to be specific about that fact. I have seen a number of their smaller knives turning up with 425 stamped on the heel of the blade, so there is some honesty there. Also, you can get these Gerbers at absolute bottom feeding prices at wal-mart, so I guess there is some value there. (Please let's not get started on Wal-mart again.)
2) They state "400 series" as a means of steamlining their advertising, packaging and literature. If they just say "400 series", they can use the same copy for everything. This would save some costs, and would be consitient with their relativily recent push for economy.
I suspect that it may be a combination of both. You can still buy a Gerber knife if you like. I for one have quite them. I do still favor their multi-tools, though. Time will tell.