Easy tiger! The question does not say it is 1019 at FL135. It says you are at FL135 and the local QNH is 1019. It also adds that air mass temp is less than ISA
QNH is a theoretical pressure at sea level, calculated as though ISA temps apply and so that your altimeter will read correctly at your datum height. In this case datum height is msl, but if QNH were given as an airfield QNH the datum height would be the airfield elvation.
So, point one. Your altimeter is measuring height from the 1013 pressure level when it should be measuring from the sea level 1019 pressure level, so you have a baro error to take out, as already explained
Point two. Even with the baro error corrected your altimeter will still be overreading if the air mass temp is below ISA, so there is a further error that brings your true height down to 13,500ft. Or, put it the other way, if you true height is down to 13,500ft the air must be colder.
The pressure at 13,500ish will be well below 1013 or 1019, more in the 600hpa region.
I have re-read this and recognise it as a long winded versio of what Charlie Zuylu has already said
Dick Whittingham