From the documentation:
The resolution of this property depends on the system timer, which is approximately 15 milliseconds on Windows systems.As a result, repeated calls to the Now property in a short time interval, such as in a loop, may return the same value.
The Now property is frequently used to measure performance. However, because of its low resolution, it is not suitable for use as a benchmarking tool. A better alternative is to use the Stopwatch class.