I find that resorts can be way off in both directions. Squaw in general over reports the amount of snow they get. I don't know about Northstar but they may be much more conservative. There was an article in Ski Magazine last year or the year before entitled "Big Lie". The article was aboit Big Sky Resort where they measure the snow in the place that gets the very least on the whole mountain and that often they report 3-4 inches and it is a foot deep on the upper mountain.
In tahoe a lot has to do with where they measure and report the snow. Many storms have limited snow at 6000 (the base of most Tahoe resorts) but may drop multiple feet more at 7000 and 8000 feet. This most recent storm was like this, and above 9000 feet there was even a few feet more. Squaw reports snow amounts at the lower mountain and also at the upper mountain and often there is a big difference. If Northstar is measuring at the base, then they will chronically underreport snowfall.
Lastly, wind blown areas can also make it look like a lot more has fallen. I have seen Squaw get 8-10 inches but in certain places where the wind has blown and loaded some areas/bowls it can be almost 2 feet deep.