Power line Voltages are NOMINAL. This is definitely the case in the US and I strongly suspect it is the case in Europe and other regions as well. There are fixed ratios between the various nominal Voltages and these ratios are determined by the number of windings on the transformers in the power grid. There is a basic 1, 2, 4 times ratio between the three most commonly used Voltages: 120, 240, 480. Oh, and for three phase, there is a 208 number in there. It is derived as the Voltage present between any two of three 120 Volt legs that are each grounded on one side. A bit of trig will show the relationship. I am sure there are multiples of this Voltage also, probably X2 and X4 (416 and 832 but they are probably rounded to something like 415 and 830)
The various power companies will have different target Voltages for the numbers in this series and for the lowest one, 120 Volts it can range from perhaps 110 to as much as 130 or about +/- 10%. This is just like the tolerances we see on blueprints (+/-0.003" for instance). The actual range and tolerances WILL vary from one power company to another.
When the load on a system changes the power plants can not change the speed of the generators instantaneously so the Voltage may drop or go up for a while. This allowable tolerance range allows this to happen and the manufacturers of electric devices MUST take these variations into account so no damage is done. Yes, a given motor will develop more HP when the Voltage goes up. And less when it goes down. This is just part of the game.
So the ranges look like this:
Low End: 110, 220, 440, 187 (208 - 10 %)
Nominal: 120, 240, 480, 208
High End: 130, 260, 520, 229 (208 + 10%)
Thus any number on the nameplate of a motor between 440 and 520 (yes, that's 520) would represent the SAME design point. A motor that says 440 should run at Voltages up to as much as 520 with little shortening of it's life. It may even run OK at Voltages that are somewhat higher or lower than this range. And none of the Voltages in the range should be considered abnormal in any way. I have a mill who's motor is rated at 220 but it has been running quite fine on 208 for years now. Not an ideal situation, but it does work and so far there is no apparent damage.
Don't get humg up on exact Voltages. If you put a recording Volt meter on the power line at any location in the world, you will find that it fluctuates by a considerable range over a period of a day or a week. I encountered an extreme example of this when I worked as a transmitter engineer at a TV station in the New Orleans area. There was an aluminum refining plant on the same grid as the transmitter facility and they would sometimes fire up an additional furnace in the evening hours. The first time I saw this happen, ther building lights dimmed severely and all my TV and audio monitors stopped working. I first thought the transmitter was off the air and I was going to try to restore operation when the people at the studio called me and asked what was happening. They said that the picture got dimmer but was still there. They were miles away and apparently the line Voltage was normal there. When I read the power meters on the transmitter I saw they were at about 35% of normal and the power line meters were reading about 80 Volts instead of the normal 120. The line Voltage slowly came back up over a period of about an hour as the power companies were able to adjust for the increased load. Yes, it really took them that long. There was nothing I could do but wait. At 80 Volts, the power was clearly out of tolerance but the transmitter still operated at this reduced power level with no damage. This included not only the electronics, but also the large blower motors the were used for cooling it. Only the monitors failed to work. Power line Voltages DO vary, sometimes by very large amounts.