To add a bit to what stu1985 says: There are two scheduling seasons every year. Northern Summer starts on the last Sunday in March; and Northern Winter starts on the last Sunday in October. Flights typically have different scheduled times in the two seasons. These can reflect broad changes in flying conditions - eg stronger winds on some routes during the Northern Winter season.

In addition, you can find that flight times change by an hour when the country at one end of the route changes to or from daylight saving time on a different day from the former convention (which is still reflected in the dates of the scheduling seasons and in European law). But this won't apply to the flights that you mention.

A couple of months or so before the start of each scheduling season, schedules do get re-jigged as the final touches are put on. However, my impression is that it's relatively unusual for a time to change by as much as an hour in this process.