The YTD shows $4,500.
Yes, he received two paychecks in April, and this one on May 15. So when he receives his final pay on September 30, he will have put in the max of $18,000.
I don’t know how teams that make the playoffs arrange their payrolls, but it appears he is receiving his entire contractual salary in 12 equal twice-monthly payments, pay out from April until September.