There are a number of reasons why this might happen:
You followed the workout, but did not achieve the step targets as planned. In simplistic terms, you trained too soft or too hard. You followed the workout and achieved the step targets yet other parameters were not as expected. The simplistic example being that your heart rate was greater than expected. You followed the workout and everything was as expected, however the system has 'discovered' a better plan than the one you were previously following.
Generally this last point is the most common, and is a mathematical / technical subject. Plan building is what is known as non-deterministic. That is, there is no simple formula to build a plan and many thousands of combinations are constructed during each plan build process and a selection of the best is made. However, we do not simply switch to the best each time, as an insignificantly better plan may look considerably different, with runs on different days being an obvious and inconvenient problem. As such we only swap to the better plan when a threshold of significance has been reached. This still has the undesirable effect that 'for no apparent reason' your plan changes, with runs becoming quite a bit longer or shorter. For people who experience regular plan jumping (more common with fewer runs per week and with crosstraining) we have a 'Plan Volatility' setting (found under your Training Settings). In essence, this option alters the significance level that better plans need to reach in order to be promoted as your actual plan - a high volatility reducing the significance threshold.
Providing information on why your plan has changed is in the R&D phase, initially focusing on the two aspects that are of prime importance to the majority of people: race-day performance, and injury risk.
And for those times when your workouts do change, but you'd really like to stick with your previous ones, you can always revert your plan.