I think this may be internal vs. external dimensions. @Homebuilt, the accepted way to measure receiver extensions is the internal depth. There can be too much variation with external length. 1/8" difference of internal depth or less can be the reason for a rifle to function correctly or not. We are not picky or bitchy but we may be anal about some terms, descriptions or measurements. It is because of the lack of standards across different manufacturers. As you have already discovered, what sound like small details can make a big difference in a rifle's performance.
Feel free to ask any questions about compatibility. It is cheaper to ask before buying, unless you say money is not a concern. We can certainly spend your money for you like a soon-to-be ex-wife.