Because humidity is a relative term. The warmer air inside your house can absorb more water vapour than the colder air outside. Outside the humidity is 49% of what the air can hold before the water begins to condence into drops of liquid, while the warm air inside can can absort more and is thus only at 25% of its capacity.
If you could count water molecules per volume, you would find a similar amount of water in the air both inside and out.
