E26 is the North American screw cap and E27 is the rest of the world. The thread form is closely related and lamps are routinely sold across both markets, which has led to a widespread assumption that one gauge set covers both. The dimensions differ, and so do the standards that define them, so the assumption is wrong even when it appears to work.
The shell diameter and the thread limits differ slightly, and the contact geometry differs more. A lamp built to E27 limits can be outside the E26 limits by an amount that is small in absolute terms but large relative to the tolerance. It will usually fit, which is why the error survives inspection, and it will occasionally arc or jam, which is why the error matters.
A test report to IEC 60061 does not cover the North American requirements, and a report to the North American standard does not cover IEC 60061. Marking a product as compliant with both without testing both is a documentation failure that surfaces at customs or in a retail audit.
If a product is sold into both markets, keep both gauge sets and mark them unambiguously. If a product is sold into one market only, gauge it with the set for that market and record which set was used on the inspection record.
