The xeno came climbing from above, I do belive getting fired upon can throw of the aim of an alien, yes. Gormans shots hit the armoured parts of the head, Vasquez did'nt (when she pinned it).
Also she pinned the xeno while it was rising, she didn't push it down, she didn't lift it, she didn't throw it away and she never carried it's weight. it was an alien that in the last 3 seconds had ben fire upon multiple times, having crashed into the floor of an airduct from some height and then goten a boot to the head... and fired upon some more...
not an unhurt xeno pouncing at a marine who's probably never even seen a xeno before... I still don't think we can use vasquez as an example here, no mather how awesome she is
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About the first shots and the acid: she probably didn't hit anything but the armoured parts. Now that probably still hurt a bit, it just didn't open up any wounds