It largely depends on what reference you're pulling from. That's why most of us have three, four, or five references to go from. What caliber and weight are you loading for, and what powder do you have availiable? Chances are, someone on here has loaded an XTP for it. Also which manual you're using would help.
One reason you'll find some bullets don't have a powder listed is space/pressure constraints. As your availiable case volume decreases, you'll get higher pressures and/or different burn characteristics. So a powder that might work for a 155grain bullet might not work for a 180, if not dangerously then efficiently.
Lead also has different characteristics from jacketed. Too hot a load and a softer alloy bullet will start to melt on the back end and lead the barrel considerably.
Also, these listed loads are tested in a lab environment with a specified set of variables. That's why you'll see the max load pressures being listed as being only recommended in the test barrel.
+1 on the post above also.