334M x 0.3% = 1,002M
The non-core pop gives something extra, but I think there is still some rounding.
Still the value is quite close.
For 0.3% you get there by 1,5% x 26% which is 0.39%. Which of course gets trunctuated to 0.3% instead of rounded up.
Paradox only uses full tenth of a percent for mobilizing, so you need a litte bit more to get over 0.4% so you mobilize more manpower.