Actual maximum service deceleration

According to the documentation:

Deceleration
A non-negative floating-point number corresponding to the deceleration in km/h/s the service brakes can provide. The corresponding brake cylinder pressure can be defined in the #PRESSURE section. The default value is 1.

However, when I tried to configure my train, I found the actual deceleration provided by the maximum service brake significantly less than the value specified. It was only around half of it. Wheel slip has not occured.
Is this value corronsponded to the amount of deceleration on maximum service brake application? If so, what could possibly cause the problem I encountered?
Will upload my train later.