The metric/imperial might be the problem. You can find out for certain if you measure the bit shank diameters with a micrometer or a dial/digital caliper. Something that is graduated in mm or thousandths of an inch.
If you don't already have one, a digital caliper like the Wixey also shows fractions if you aw within .003" of a fraction. It can be switched from mm to imperial with just the push of a button too. I would not want to be without one in my shop. It can be used to measure board thickness, board length up to about 5", hole diameters, and hole depths very accurately.
About $40 US.
Wixey Digital Calipers with Fractions Model WR100 - Product Info
wixey.com
Charley