Different parts of the world have allocated spectrum that's in use for other purposes, so the operators have to deploy LTE in the parts of spectrum that are ava

Different parts of the world have allocated spectrum that's in use for other purposes, so the operators have to deploy LTE in the parts of spectrum that are available and they have a license for. All these(700/800, 1700/1900, 2500, 2600) are standarized bands as of today, though different bands have been standarized at different points in time. This will be less of an issue as time goes on, newer phone models will support the various frequency bands.