Nominal means "in name only" and has little to do with the actual impedance of the speaker. That’s why it’s confusing. The speaker impedance is measured across the frequency band and someone at the speaker company looks at it and says hmmm... that looks like about 6 ohmsish on average so we’ll call these 6-ohm speakers.
This is one area where published specs are important. I look at the impedance curve of my speakers and they are a flat 2.5 to 3 ohms from 100hz to 10kHz which tells me I need a hefty solid state amplifier for the low impedance but a tube amp with a 3 ohm output tap would work great also since the impedance is flat.