Hm mathematically it should not be possible.
I'm not sure that's the case. Personally, I think that phase missiles ignore mitigation (see below).
But even if they don't, the maths examples people have given aren't taking into account regeneration of shields and hulls.
Shields regenerate quicker than hulls, and continue to regenerate even once they're knocked down. This alters the effective DPS you inflict. A (purely hypothetical in order to demonstrate what I mean) example:
Suppose the target ship has 100 shields and 100 hull. Say it regenerates shields at a rate of 5 DPS, and hull at a rate of 2 DPS.
If you attack with a ship with 10 DPS and no phasing, it'll take:
20 seconds to take down the shields (10 DPS damage - 5 DPS shield regen = 5 DPS done, 100 shields / 5 DPS = 20 seconds)
33.3 seconds to take down the hull (10 DPS damage - 5 DPS shield regen - 2 DPS hull regen = 3 DPS done, 100 hull / 3 DPS = 33.3 seconds)
Total: 53.3 seconds.
If you attack with the same 10 DPS ship, this time with 40% phasing, it'll take:
100 seconds to take down the shields (60% of 10 DPS is 6 DPS to shields. 6 DPS damage - 5 DPS shield regen = 1 DPS done, 100 shields / 1 DPS = 100 seconds)
50 seconds to take down the hull (40% of 10 DPS is 4 DPS that ignores shields. 4 DPS damage - 2 DPS hull regen = 2 DPS done, 100 hull / 2 DPS = 50 seconds)
Total: 50 seconds to destroy the ship (which will still have half shields at the time).
So, even if phase missiles don't ignore mitigation, they can still be more effective than normal weapons.
Perhaps hull has damage mittigation also and missles bypass that to..
Mitigation happens whether or not a ship has shields left. This is explained in the manual, and quite clear if you watch the info cards of heavily damaged ships - they still show mitigation even when the shields are down.
As mentioned above, I'm of the opinion that phase missiles bypass mitigation when they successfully 'phase', but even if they don't, they can be more effective than non-phasing weapons.