Undead creatures usually don't apply their Constitution modifiers to their Hit Points, usually because they don't have Constitution scores. Some apply to their hp another ability score's modifier (for example, the charnel hound (Monster Manual III 26)), but these are rare and not usually suitable for PCs.
Thus, for example, a PC wizard with 12 Hit Dice and a Constitution score of 8 would normally have 4+11d4−12 hp (that is, on average, less than 20!). But, when that PC wizard gains the type undead and its Constitution score becomes
—, the wizard has 12+11d12 hp (about 83 hp!). (Note that while monsters don't get maximum hp on their first Hit Die, PCs do.)
Other effects can modify a typical undead creature's hp—such as creating it in an area affected by the spell desecrate that also contains an altar to an evil god—but such effects are usually barred to the typical necropolitan.