Because the denominator decides the answer. The same team, same year, produces five defensible percentages from 7% down to 0.9% depending on what sits under the division line. Anyone quoting the 5–10% rule without stating the denominator is quoting noise.
The popular "5–10% of sales and marketing spend" only survives if systems are counted alongside people; on a people-only denominator it lands at 4%, and at a heavily-funded company nearer 3.6%. Two large datasets also disagree by 2.7× on staffing ratio — 4.3:1 versus 11.8:1 — because one surveys VC-backed companies and the other scrapes titles market-wide.
Every claim above comes from work LeanScale published. These are the sources.