Uneven tile grout color is not normal but there can be a number of things that can cause blotchy tile grout. Also how uneven the color appears can be somewhat subjective. Certain color tile grouts are not as easy to control. The easiest colors to work with are the medium grays and beiges as opposed to bright colors, whites, blacks, etc. Since most tile grout is cement based it tends to turn gray with time and cleaning. Tile grout sealers are now recommended to reduce this effect.
Some of the things that can cause color variation include:
1) Too much water added in mixing the tile grout.
2) Improper cleaning of the tile grout and grout haze off the tile (usually by wiping too soon and with too much water).
3) Leaving plastic spacers in the joints,
4) Having the thinset adhesive fill part of the joint and showing through.
5) Improper mixing of the tile grout by using an improper amount of water
6) Mixing partial bags (this is a problem if the colorant is not evenly distributed).
7) Using tile grout from a bag that was previously opened (and gained moisture during storage).
Adding water after the first mixing (rehydrating).
9) Failure to slake the tile grout.
10) Improper joint packing.
11) Inadequate cleaning of the sponges.
12) Using unclean water.
13) Defective tile grout itself (this happens once in a while).
14) Dirty tools or shoes that stain the tile grout (we have actually had that happen to us, where workers tracked in clay on their shoes).
15) Foot or other traffic too soon over the joints-curing time is on the bag but can be up to 3 days for some tile grout.
Sometimes the tile itself can have a microporoisty (small pores in the tile body, especially with porcelain tile) condition that sucks the moisture out of the tile grout too fast or in an irregular pattern.
In many cases it is hard to tell exactly what went wrong by looking at the tilework. I would suggest that you ask the tile installer what he or she thinks about the workmanship and techniques used. One can always remove the tile grout and replace it if that is the only acceptable solution.