Another spark plug question.. winter..

Hello folks..
I got a 2011 grizzly 700 eps that in the NGK list find me a xxx6xx plug. Think my service book says a xxx7xx plug..
Anyway-, got me the xx6xx plug and went on driving this winter. But now when its getting colder,like under zero celcius and some mornings -12 degree celcius the bike needs a couple try to start. Can that be the different number in the plugs "heatnumber"?
Or is it just normal to be a bit slow starting in the cold whatever plug i use?
Runar
Norway