In an ideal situation there should be some clue that you might be able to pick up from the analytics, although they may not be significant enough to give you a solid starting point.
If you had access to a large number enough of Android and iOS users, then the first thing might be some basic usability testing to get an understanding of the different user attitudes, expectations and behaviour.
Ultimately it might just be that the e-commerce site's design or products/services are more appealing to iOS users. It would also be interesting to see a comparison between Android users that rate the website higher than those that rate it lower, as well as compare the range of usability and customer satisfaction scores between Android and iOS users.
That's probably as much as can be said without looking at the actual analytics but hopefully there are a couple of starting points to do further research.