well, Cron ,it's something like this:
thessur it's 1030,higher than 1013 ,right.So,the real pressure altitude it's lower,cause the pressure decrease with increasing altitude.
1030 it's lower than 1013.Imagine you move down on a ladder to the step marked 1030.
The difference can be calculated .1030-1013= 17
Then 17 * 30 =510 (it's aprox 30' for each HPA)
Thus resulting 510' difference with a - sign,as I told you above
Alex