In Britain, it rains on average every 1 in 3 days. Therefore to maintain a reliable service, we will likely visit on days when it rains. If we do rearrange, due to weather, that is so we don't get wet!
Why does it work?
The pure water we use on your windows dries without leaving marks or smudges. The relatively small amount of rainwater that would hit your windows doesn't affect this.
Rain water is naturally distilled by the weather cycle so it is often pure water or close to pure.
If rain falls on already dirty windows, it mixes with the dirt, then dries, leaving water marks. However if rain falls on clean windows it evaporates off, leaving a clean window.
This means that skipping your regular maintenance cleans, even in wet weather, will lead to dirty windows. A regular clean is the only way to keep your windows clean.