It's because the tension drops a lot, especially in the first week or so after stringing. Some strings hold tension better than others, but all strings drop tension over time, even with a perfect stringing job. Tension loss happens even if the racket isn't used, but it will happen faster if you play with it a lot.
They say smashing with a higher tension is harder cause there's less propulsion, but in my experience; smashing with a high tension like 28-30lbs makes my smash better.
Smashing with higher tensions is indeed difficult for several reasons. For someone who's probably already used to higher tensions, you would not have noticed. Your smash is better because your power is sufficient, the control and the feel of the sound of a high-tension racket. For people who aren't experienced enough, Using a higher tension will not generate power as expected since the power of that individual can not match the power generated by the quick repulsion of that high tension. You would call that a 'trampoline effect,' meaning the higher the tension is, the less time the cork hits the string hence strong power is required to generate powerful and accurate shots. Also, higher tensions decrease the sweet spot area and increase the risk of the string breaking (or even the racket).
