Often because a large share of that volume is not about your subject at all. The clearest example we measured: the phrase turkish breasts returns 49,500 searches a month, and turkey breast returns 49,500 searches a month. The same number, because the planner is folding the demonym into the bird. People shopping for poultry are being counted as your audience.
This was not a rounding issue. Across one research set of twenty groups by fifteen descriptive terms, the raw total came to 349,590 searches a month. Of that, 156,200 was poultry. Nearly half the apparent market was dinner. The same collision showed up on legs, neck and hands.
Decontaminated, the real figure was 169,690, and once we looked at what was left, 73 percent of it was head and face terms that existing pages already covered without generating anything. The genuinely new opportunity was a small fraction of what the first spreadsheet showed.
The check is cheap. For any high volume phrase in your plan, search the non-adult reading of the same words and compare the numbers. If they match exactly, your volume is not yours. Do this before the plan gets approved, not after the pages are built.