I have a question

I think you mean that it is WS 3 on all those things. I would guess it is a typo. There is no reason an ungro charioteer should be WS 4 instead of 3. GW doesn't do good proofreads over any of the things they put into print. I just recieved my Empire in flames book for mordheim and in the beastman skill chart the number three is there instead of a check mark. I'm sure it was just a font mess but it still should have been caught and not printed as such. I've worked for a printing company and most businesses review the first one off the press to make sure it is what they want.

There's no reason to assume that the Ungor in the Chariot would not have Ws4, if it is said to have it in the book. Charioteers may very well be elite. Look at Tomb King chariots. The Skeleton Warriors have Ws3.

It is the same for cavalry, look at say for instance, a Saurus. It has WS3, but Saurus cavalry have WS4. Dark elf warriors have WS4, Cold One Knights have WS5. Generally, models on horseback or chariot are probably there because they are elite warriors. They wouldn't just throw on any old guy. (although I don't know why they didn't just put a Gor on the thing. They are better anyway...)