The simple answer to this is question is, the increased temperature means that one side of the brake system is doing more work than the other side which is resulting in the temperature difference.
The reason why this is occurring would require detailed inspection of your braking system, but normally this is due to increased friction or insufficient friction balance, it's probably natural to assume the side getting hot has the problem but it's also probable that the cooler side has the problem which is making the opposite side work much harder than normal to reduce the speed.
Our advice would be to inspect both brake calipers for any defects, this could be torn piston seals, worn slider sleeves or sticking caliper pistons. Pistons that are not able to move freely will result in brake binding which will cause the pads to rub against the discs constantly causing higher temperatures than normal, uneven braking and eventually damage to the brake discs not to mention increased wear on your pads.
So if you have concerns that one side is getting hotter than the other please get the brakes checked by a reputable professional, preferably the mechanic should belong to a professional register such as the IMI or be ATA accredited to illustrate they have undertaken professional training and are continually developing their personal development.