mcdhu:
If the controller wanted to provide 1000ft separation against something flying at 6000ft on the QNH, then he/she could have cleared you to an altitude of 7000ft. The fact that the QNH happened to be 1013 makes no difference. A similar situation can arise in times of low pressure (i.e. QNH less than 1013) when the lowest available flight level which gives at least 1000ft separation against 6000ft might be FL80. But if you are climbing underneath traffic at FL 80 you can be allocated FL70. It is separated from the one above but not of course from anything at 6000ft underneath.
I used to have this argument regularly with people who used to say: Low pressure, so FL70 does not exist! Rubbish
