There is some information missing here that could help nail the issue down. First, how are you measuring the mechanical advance? Are you using a dial-back timing light? Using a degreed balancer? Using a distributor machine? Using timing tape? Regardless of how you are measuring the 10 degrees, if you are using a distributor machine the 10 degrees is 20 at the crankshaft. If you are measuring on a running engine in the car, you could have very weak or very strong advance springs. Very weak springs may be giving you considerable advance at idle you are not accounting for. You didn't mention idle speed for your starting number. Very strong springs may only give you 10 degrees at 4000-4500 RPM,s or whatever speed you are assuming full advance is happening at. Fill-in some of the blanks and people will be able to help you. It's always possible, there is something mechanical wrong with the aftermarket distributor, wrong weight shape, advance slot and so forth.
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