OP description is a little confusing because it compares speed on the rollers to speed on a "stationary bike". Unless "stationary bike" actually means riding the same bike on the same rollers, any comparison is pretty meaningless.

Now, if it is an apples-to-apples comparison, then the improvement may be attributable to the effects of a "taper". Modern approaches to training model stress (acute training load) and fitness (chronic training load) using exponential decay formulas. Training stress balance is the difference between your current stress and fitness. Since stress decays much faster than fitness, extended recovery will increase your training stress balance resulting in a (possibly significant) performance improvement.

Of course without detailed tracking of all your training, all we can do is speculate. But for someone who has a pretty consistent training load week after week (with no periodization), what you observed is pretty typical.