{Vorel of the Hull Clade} does this ability double the number if counters on a card?
Yep! From Gatherer:
4/15/2013 The effect of Vorel's ability will essentially double the counters on the target artifact, creature, or land. For example, if a creature has three +1/+1 counters and a divinity counter on it before the ability resolves, it will have six +1/+1 counters and two divinity counters on it after the ability resolves.
Is there any reason (probably a very particular one) why it doesn't just say "double"
If you have 3 1/1 counters and a time counter on a creature and you double the amount of counters, it's possible to misinterpret it and put 4 1/1 counters so that you now have 8 counters.