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Determined by the market a.k.a. the effect of many decisions made by many brains. :)

If you put in 1 ETH during one of the 23h windows and now one else sends any ETH that day, you get 100% of the EOS tokens.
If you put 1 ETH and multiple people put in a total of 99 more ETH that day, then you get 1% of the ETH for this 23h time slot "auction". Since 1 ETH divided by 100 ETH = 1%
Had only 49 ETH been added that day, your 1ETH would have purchased you 2% of the daily EOS tokens.
Makes sense?

then it is better to wait for the second or third window? cause you get it cheaper?

That OR more people will know about it and it'll be more expensive. If we knew the answer to that question we'd all be whales d: