This is totally a guess because most of your files are on AJ and it’s a wildly different marketplace than the others – but from my experience that’s probably your release cycle more than anything else… New products and frontpage exposure will create spikes during the 1-2 weeks that you launch new stuff. So if you release more early in the month, even just one popular file can make things seem uneven.
That, and normal fluctuations or outside blog exposure can create artificial peaks and valleys until you get enough products up to even things out (edit – with 64 products this isn’t likely the case where it might be a valid argument for a TF author with just 3 themes).
The only trend that I can really set my clock to is that Monday – Thursday always sell about 20-30% more per day than Friday – Sunday 
Edit 2: this isn’t an official staff response btw despite the badge (I work with WPTuts, not the marketplace) – it’s entirely based on my personal experience on here. 

Oh hey, that’s me! Drop me a line at the product page or at our official support center: