Hi Max, good catch. This comes from SDPWS 2021 Table 4.3A, Footnote 10, not from 8d nails being inherently stronger.
When overturning tension is resisted by a hold-down on the inside face of the end post, the code requires the nominal unit shear capacity for walls sheathed with 10d common nails to be multiplied by 0.92 (an 8% reduction). That factor does not apply to 8d nails.
That reduction reflects cyclic test results: eccentric inside-face hold-downs can cause prying and nail damage at the corners with 10d nailing. It typically does not apply for hold-downs on the outside face (or both faces), or for continuous rod systems.
In the calculator, that footnote is applied whenever there is chord tension (T_chord > 0) and the selected sheathing uses 10d nails. So for the same panel thickness and edge spacing, a 10d option can show a higher utilization (lower effective capacity) than the matching 8d option, even though the raw tabulated 10d values in Table 4.3A are higher.
At wider spacings you’ll often still see 10d come out ahead (the tabulated increase outweighs the 0.92 factor). At tighter patterns, the footnote can tip the comparison toward 8d, which is what you’re seeing.
Happy to walk through a specific sheathing row if useful.
