The mind and body can play tricks. The deceleration to felt could well have been accurate - most likely at ATC request for en-route seperation or arrivals sequencing. Passenger aircraft are designed to be as efficient as possible, and that means minimising drag. In addition, the amount of kinetic energy involved in >50T of metal, fuel and flesh travelling at 500kts is significant.
What this means is that a deceleration of 20kts IAS or more could take a minute, or longer. During that period, one's body can adjust and perceive the deceleration as the new norm. When the target speed is achieved, more power will be required to maintain it (although typically not as much as the previous cruising speed), so the engines will spool up as required. This can feel like an acceleration - partly because one's body had created the new normal of a roughly constant deceleration rate, and because our minds associate an increase in thrust with acceleration. Furthermore, the new speed will require a higher pitch attitude than before, with more of one's weight acting through the back of the seat, rather than the bottom. It's a small change, but it serves to strengthen the illusion.
The most extreme examples of this are a descent with flap out at constant speed to level flight. If the level off coincides with a turn - and thus increased lift required, induced drag and,as such, power required; the resulting pitch change and increase in thrust can feel very much like the chaps/chapesses at the front have decided to "step on it".