If a lens spec stats 100mm, just how accurate is that? Two aspects of this question apply. One is the design, where the actual value might be, for example 101.2mm and it is just marketed as 100. The other aspect is unit to unit variation. I'd expect the latter to be quite small, more so for the better lenses.
It's kind of like people saying 30 frames per second for NTSC video, which it's closer to 29.9700299700299y(...) and is exactly 30000/1001 ... within the precision of the oscillator used to drive the video (which is usually very accurate ... within 0.00028% for TV broadcast under the old analog standards in USA ).
Things like electrical components, resistors and capacitors, tend to have a few percent variation. A 1000 ohm resistor may really be 950 ohms. But with lenses, I'd think if there is that much variation in glass, different elements would not match well together.