Your initial value is too large and increment of one in this case is impossible.
By default, matlab use double precision floating point, and there is limitation of floating point representation.
Floating-point are saved in exponential way, so it can represent very small value (precise number) at one time, and represent very large value (large range) at another time, but both large range and precise number can't be obtained at the same time.
When you add a very large value with a very small one, the small number will be neglected.
If you want increment of one, try smaller initial value.