This might be "normal", depending on how the signal is acquired. Whenever a signal is derived, timing of the signal acquisition is crucial.
Counting ticks is not time sensitive, as it does not matter if one tick is acquires a few microseconds later or earlier, what matter is, that it is acquired.
For deriving a velocity from the ticks of an encoder, we need to know exactly when the tick happened. Given a constant velocity of 100 mm/s, we can assume a wheel circumference of 100mm, meaning 1 rotation per second. If an encoder has 1024 ticks per rotation, translates to approximately 1 tick every millisecond.
So if we do the calculation backwards, 1024 translates to 100mm/s, to make the calculations easier let us assume 1 tick translates to 0.1mm movement.
1 tick every 1 millisecond translating to 0.1 mm movement every millisecond.
In order to calculate the velocity for every tick, means that we need to measure the time between ticks, which translates to registering the exact time of the tick event or stating a timer and stopping a timer at tick events. The precision of the time measurement can be influenced by:
The timer precision. If the timer has a millisecond precision, that, in an extreme worst case could mean that we have 0 registered milliseconds between two ticks resulting in an unfeasible velocity. A 0.1 millisecond precision of the timer would mean a 10% precision of the velocity for the example above
The event detection. Before we get to start top timers or register time, we need to actually detect the event. If the encoder can trigger hardware interrupts, that translates to starting the interrupt routine at clock frequency (e.g. 8Mhz for a microcontroller). If we are regularly polling the state of the encoder, the polling rate has a major influence on the precision. If we are polling with 0.1 milliseconds the extreme worst case is that we register the tick event 0.1 ms late, but we register the next tick exactly at the right moment. So 0.1 ms polling would lead to a 10% precision on the velocity for the example above.