Ebay doesn't have to explain why they hold onto a payment for two days after tracking proves delivery - it's their site and their rules which you agree to by selling on the site.
Currently, if items are sent untracked sellers are paid 14 days after the transaction.
However, as explained, regular and reliable sellers will be paid more quickly from 6th August.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)