What's the difference between 12V and 18V panels?

It's a bit confused about some of the stats on panels we have been looking at, for example, 100 watt 12 volts panel and 100 watt 18 volts panel. In the majority of cases there are no differences other than name.

In the early days of solar panels they tended to be small and often were just directly connected to a 12V battery for charging purposes.

Even though the open circuit voltage of these panels was much higher than 12V, directly connecting to the battery made for an inexpensive charger. And since the battery was 12V it was easy to think of the panel as also being 12V.

The true maximum power point of these panels (and most modern 12V panels) is close to 18V and thus should be considered 18V panels not 12V.

Also, most panels advertised as 24V are really 36V or two 18V panels in series with an open-circuit voltage well above 40V.

Both 12V and 18V panels are listed for sale on Amazon and inspection of the electrical specs shows that they are essentially identical.