It boils down to the issue that the MidHeaven and the Ascendent are hardly ever exactly 90 degrees apart. With the Angles bent out of shape like this, it's impossible to get equal 30 degree houses. The reasons for this discrepency are strictly geometrical, again. Generally speaking, houses tend to approach 30 degrees near the equator, but become really distorted as you move towards the poles. The exact details are unimportant, however.
Some systems seem to make the inequality of house sizes even worse.
In general, unequal house sizes are simply a fact of life on planet Earth. Occasionally with a big house, it's large enough to completely swallow up an entire zodiac sign. Such a sign is called an Intercepted Sign . The opposite sign of Pisces is also intercepted. An intercepted sign and any planets in that sign represent a kind of hidden or repressed energy, since that energy doesn't have a direct path through a house cusp in order to express itself.