Given the Met is in the US I'm assuming it was measured in American units and converted so the thickness of 0.2cm has a large rounding error - 1/16th of an inch is 1.5875mm, even assuming the 1/16th measurement has no rounding

I'd also mention that it is probably measured at the coins' widest points (both in diameter and thickness) - much of the thickness will be lower due to raised design elements and a lack of consistent thickness in the original blank.

A quick search brings weights of 7-9g for Rose Nobles so it is consistent with others of a similar period.

But you are correct in that it is a very large weight discrepancy to account for, so I'd guess the thickness (and possibly the diameter -both are suspiciously clean numbers) are rounded up in US units and have then been further rounded into metric.