Why's it commonly believed that MOST women can't drink as much alcohol as blokes?

Allow me to shoot that theory down in flames! A few years back, I hit the pub with a mate of mine and his girlfriend one night, they more or less matched each other drink-for-drink ALL NIGHT LONG, and, although, granted, me mate's girlfriend was a little bit worse for wear by the end of the night, she still got up and walked out to the cab under her own steam, but my mate went out like a light- NO word of a lie! (I mean the silly sod was out bloody sparko- ding-ding, out for the bleedin' count!) It took THREE of us blokes to get the silly bugger out to the bloody cab! Now, apparently, his girlfriend woke up next mornin' and she felt nigh-on right as rain, but apparently, my mate, on the other hand, felt like absolute ****, (and I mean he told me he felt bloody terrible!) Now, why's that, if womens' bodies are SUPPOSEDLY A LOT more intolerant to booze than blokes' bodies are? And no, before anyone asks- me mate's girlfriend WASN'T of a large build. In fact, there was sod-all of her- she was as skinny as a rake! I'd REALLY appreciate YOUR angles on this one, please, guys! Much obliged in advance! Peace and love! Be lucky! :)