I apologize for any confusion. Arcadia is standardizing products for the US and Canada. According to Arcadia:
To stay within ETL, which the fittings now have! In the US and Canada they have to state E26. This is a 26mm thread width.
Lamps can be E26 or E27 and you can legally use E27 in E26 holders.
E27 in E26 is tight and safe. E26 in E26 is perfect. E26 in E27 officially could be loose.
We are trying to now standardize all 110v product to E26 where possible.
John Courteney-Smith MRSB FLS
Head of Science and Innovation for Arcadia Reptile and Bird
I hope this clears things up to some extent.