Meshed backed mosaics are notorious for irregular spacing. The individual tiles are placed face down in a either a grid, or perimeter outlined jig then a glue dipped paper or nylon mesh is applied to the back and let to dry. Occasionally the tiles are uneven themselves or simply a bit smaller than the grid or jig and fit a little loose resulting in shifted lines. Even if they are initially set straight the netting can stretch and misconstrue. Finally production control, shipping and storage that can affect the glue which can cause the tiles to come loose and be slightly offset. These conditions can sometimes make consistent joint spacing painstakingly difficult or even impossible in some cases. The contrasting grout color you chose for this application only highlights the imperfections. It's the manufacturers responsibility to provide a quality material. The tile installer is simply the end user with the responsibility to inspect and install the material with minor adjustments within reason to fine tune slight irregularities. This tile setter did inspect the material and made you aware of the imperfections. It is possible to cut the tiles loose and readjust them on the wall but that may become very labor intensive and run above and beyond what is his responsibility at standard pricing for installing the tile that is supposed to be install ready upon arrival. That being said 99.9% of all tiles are imperfect and tile installing is an art of averaging imperfections. A good, detailed installer will regularly account to go above and beyond what is traditionally expected because they know what it takes to make imperfect tiles look "right." You will however, pay a premium price tag for this.