ah, it's because it doesn't work like that, really. you've kinda got a shorthand version.
basically, as i understand it, the deal is that at some time between 17 weeks and 26 weeks-ish, everything falls into place for eating. gut matures and you're good to go. but how do you know when that's happened? well, you don't, really, which is why it's smart to keep to the outside figure. by 26 weeks or so, they're all ready to eat. some might have been ready for weeks, some for a few days, but you're not getting children at that stage who aren't ready. do you see? it's a bet-hedging thing.
now, as it happens, having waited patiently until 26 weeks with dd1 (well, nearly, she grabbed a peach out of my hand a few days before) i was a bit more chilled about it with dd2. she was prem, and the advie with preemies is v confusing, some docs say wean early, some late. however, i figured that when she grabbed food out of my hand i'd let her take it, leave the decision up to her.
sure enough, at five months corrected, six and a half after she was born, she swiped and licked and chewed. took a couple more weeks until she swallowed and then she took to it fine.
so i guess what i'm saying is that you're coming to it a bit ass-backward. the GREAT thing about blw is that if they're not ready, they simply cannot eat it. they'll lick etc, maybe, if they can hold the food for a while, but basically all the ducks have to be in line before they can get started. and it would be nuts to think that the human body was designed for their backs, hands, fingers, mouths and tongue etc to be ready to eat, without having first sorted out the stomach.