I agree that 20 to 28 thousand dead when there was a total of 45 thousand troops total engaged seems more than a little high. I believe that the numbers of dead include a great number of people who died in the days to weeks following the battle from wounds that went septic, or other complications.
In the US Civil war it was not uncommon for there to be an indecisive battle and for each side to have circa 10% of their troops be casualties. The percent of the wounded who lived to fight another day has increased as medical knowledge and the ability to apply it has improved.
"On the other hand, for Lee, who was massively outnumbered as usual (65,000 men to Grant's 123,000),"
Forces Engaged: 162,920 total (US 101,895; CS 61,025)
Catton, Grant Takes Command 2,265 (killed) this is the highest estimate for Union soldiers killed in this multiday battle that ended with the woods catching fire and the wounded soldiers being burned to death--which seems comparable to the snow at Towton.
but note the low number of deaths compared to the number engaged.