If you're talking about the occasional ~40m AP item while your alts only ever get ~20, then as Aster suggested it's probably the tech upgrade.

If you're talking about many items being worth, say, 18m on one character and 20m on another, then you probably don't actually have AK40 on the toon getting the lower amount. Check your AK level and if it's not 40, check to make sure you don't have any Notes or anything sitting around.
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