Ok, here's how the rules *actually* work...
Skimmers (Not fast vehicles, and not just Fast Skimmers), all skimmers that move over 6" (not over 12" just over 6"), treat all penetrating hits as glances. p69 of the main book, under the gigantic heading "Skimmers Moving Fast" Hpwever, if a skimmer is Immobilized while moving over 6" it crashes and is destroyed (p67)
Fast vehicles (all fast vehicles, not just skimmers) may move up to 24" in the movement phase (p61), and they may fire one main and all defensive weapons if they move up to 12", and fire all weapons if they move up to 6" (p63)
Not all Skimmers are Fast, nor are all Fast vehicles Skimmers. However, Fast Skimmers are very frustrating to deal with because they have the ability to fire even after moving fast enough to gain the protection of speed. A normal skimmer like a Tau tank CAN move over 6" and gain the ability to treat penetrating hits as glances, but if they do that they cannot fire, since they are not Fast. (though they can buy a piece of gear that lets them fire as if they were fast, thereby gaining both benefits)
So, that's why people complain about skimmers being hard to kill.
If they're moving properly, no matter what you hit them with it only has a 1/3 chance of killing it (since only a 5 or 6 destroy a skimmer moving over 6"), but if they did not have this rule, then a penetrating hit would have a 2/3 chance of destroying a fast moving skimmer.