Just to clarify: a T(lag) of >= 2.5 hours lies somewhere in the 2%-5% range, per experimental study data, not 0.5%.
And even at the low end of this margin of error, that's 1 in 50 people, so it's rare but by no means impossibly rare. And as has been pointed out before, of the 1 in 50 with T(lag) >= 2.5hr, around 90% of such people have a T(lag) in the 2.5hr-3.0hr range, another c.8% in the 3.0hr-3.5hr range, another c.2% in the 3.5-4.0hr range, and none in the >4.0hr range.
Which means that it's highly likely that Kercher died some time between 9.00pm and 9.30pm, that she almost certainly died before 10.00pm, and that it's effectively impossible that she died any later than 10.30pm