Why the voltage of electric current is measured in multiple of eleven

I assume you mean the US power grid at 110V (actually closer to 120V) and the Europe one at 220V (sometimes 230V)?

It was just decided that way, there is nothing special about the multiple of eleven.

It's not always that way, in San Francisco the trolleys run on 600V lines. In the UK we use 240V. Plus the examples I listed above.

Once, for example, in the US they decided to have 110V as the main voltage, it's easier to build transformers that multiply this voltage up, which is why high voltage lines are still powers of 11: 22kV, 33kV etc.

The pattern of multiples of 11 isn't really there, it just looks like it is.