Because of max and min levels.
The highest level 39 necro pet is let's say... Level 35, the lowest level for pets to DW.
The lowest level 44 necro per is let's say... Level 34, too low to DW.
All pet spells have a range of several levels about 3-5 level spread. The pet could be anywhere from 32-35 for example. If you hit the high end of the range it DWs for the level 39 pet spell. If you hit the low end of the range it doesn't DW for the level 44 pet spell.
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