1866ghz RAM and 1867ghz DDR3 Ram are they the same? Apple specify 1867ghz DDR3 for the new 27 inch iMac with 5K Retina. Not sure if supplier is telling the truth?

I want to buy some more RAM for the new 27 inch iMac i7 with 5k Retina and Apple specify 1867ghz DDR3 but Mr Memory (here in the UK) are telling me their new compatible RAM at 1866ghz is the same and it's just a rounding down issue i.e. 1866 is actually 1866.666 recurring, in which case isn't the Apple 1867 therefore 1867.777 recurring? And Crucial memory don't appear to know. Also, when I look on the USA site of OWC they specify 1867ghz as well. So I'm not sure whether I'm being told the truth and as I'm not a 'techie' I don't even know if it actually matters?

Any advice please??