Why do they vary though?

Presumably based on the different lengths of growing cycles for different plants.

There may be more tomatoes ready for harvest this week, and more kale ready next week.

And customers may request more or less of certain things at different times of the year.

I'm sure they try to match harvest schedule and demand as closely as possible, but plants and customers don't always operate in a perfectly predictable manner.

To add onto what the other commenter responded with, Lufa doesn't just provide their own produce from their greenhouses. They also supply local produce and products from all around the region, including going as far as Lac St Jean region and Eastern Ontario for stuff that is made locally. Some is greenhouse, some is seasonal.

Outside of this, it sources citrus from small growers in Florida for example.