The problem in Mexico is the gross misuse of Ohms Law. Ohms Law says that I=E/R, where I = Current (Amperage), E = Voltage, and R = Resistance (Load). Which shows that, for a fixed R, I will drop when E is increased.
The amount of current (I) that a cable can carry is determined by the resistance of the cable. This is set by the size of the cable. For Alternating Current (AC) systems this means the amount of copper in the wires.
A lot of the cables in existing power systems at the consumer levels are too small for the current they need to carry; caused by two things; 1)the wholesale theft of power by large percentage of the population and 2)the rise in consumer electrical/electronic hardware.
According to Ohms Law if you increase the current on a cable with a fixed load (R) the voltage decreases (and the conductors in the cable will start to heat up). At a certain point the drop in voltage starts causing brownouts and burning up motors.
To prevent this and also to increase the amount of current a cable can handle, the power companies increase the output voltage. However this isn't a cure-all, You still end up with a massive voltage drop over the total length of a circuit. So you start out with 137 volts but, at the end of the circuit you have less than 107 volts. But this is still easier than kicking all the unauthorized users off the circuit and cheaper than running bigger cables.
This is why you will find both high and low voltages in Mexico.
But Mexico isn't the only country with a under-built grid. The U.S also has this problem, it's just not as bad. The power companies here don't jack up the terminal voltage, but they also cannot handle the maximum peak loads that the systems can generate. Hence the planned rolling blackouts in the major cities during heat waves. But this isn't a problem with the local grids (from the transformer on the power pole to the furthest point on the circuit, the voltage will be within the acceptable range of 108-127 Volts). it's a problem with the national high-voltage grid and the total capacity of the generating plants.
