My wife is also an RN and does the same thing–actually my wife pronounces it as “sahnt-o-meter.”
When I asked her why she does this, she insists that it’s the correct pronunciation because the word comes from French. My response has always been that the word is now totally English, and we don’t pronounce other words (like restaurant) that come from French in an exaggerated faux-French manner. Nobody else in the scientific community in the U.S. pronounces it this way, including chemists and physicists who use the unit routinely.
Personally, I think it’s a hypercorrection that has spread through the nursing schools in this country. Right out of the wikipeida article, hypercorrections like these are “often combined with a desire to seem formal or educated,” which I attribute to an inferiority complex that some nurses have around physicians.
Some of the other medical professions (including physicians, who should know better) have picked it up from the nurses.
If nurses are now pronouncing it as “sahn-o-meter,” dropping the “t” sound per the OP, then the hypercorrection is getting worse instead of diminishing.