I'm surprised no one has mentioned the use of different grades of aluminum alloy used in the tubes yet.
The majority of commercial tubes are made from 6061. The "better" mil-spec dimension tubes (not the $20 ones) are made from 7075-T6.
Tensile strength of 6061 = 45000psi
Tensile strength of 7075-T6 = 83000psi
That is a substantial difference the way I see it.