That's normal. The reverse could just as easily be true on your next purchase.
Surplus WC844 will have been purchased in bulk. Bulk purchases are not blended to a standard burn rate the way canister grade stuff sold to handloaders is. The expectation is the loading manufacturer will use a pressure test gun to adjust his load, but the handloader will be stuck with published load data that doesn't vary, so the handloader needs a more consistent burn rate, even if it costs more to achieve that (and it does, which is why large scale ammo manufacturers buy in bulk and determine their own load data).
Hodgdon has H335 blended to ±3% of a nominal burn rate value where some bulk purchases can vary up to ±20% (though 10% seems to be more common). Anyway, if the seller is telling you H335 load data applies, then that should mean he tested it (and did not assume that) and found the minimum data was be safe. Data beyond that may not be safe or may still be underloaded, as you found out.
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