All I can say is that you're not reading it correctly.
$19.2M in signing bonus is what is being charged against the cap. This is not what he receives in signing bonus this year.
In the spotrac link, the last column is yearly cash, which is $35M for 2019. This reflects the base $5M, and $30M of the signing bonus.
https://www.spotrac.com/nfl/seattle-sea ... lson-9885/
RW's old contact - if you scroll down on the spotrac page - had a base salary of $17M and a prorated signing bonus of $6.2M for 2019. For salary cap purposes the $6.2M gets added to the new contract's prorated signing bonus of $13M. The 2019 charge against the cap for the signing bonuses is the total of $13M + $6.2M = $19.2M.