Distribution of phase errors
Use the normal gaussian distribution, there are very few (physical) phenomena that have uniform distribution. I mean, if some device is designed for a 90 degree phase shift, in practice it will have some variation. This variation will have a normal distribution. For example the chance of having a 90 degrees phase shift is the largest and the chance of having a 91 degrees phase shift is 0.01 %.

A uniform distribution says for example that the device has an equal change of giving 90 degrees phase shift as it has of giving 91 degrees phase shift. But ZERO chance of 91.001 degrees phase shift. Think about that, it's not physically possible.