The answer is that
- They don't test on all of the combinations
- They focus on the ones that are used the most
For example there are literally thousands of Android devices and sizes and OS versions. So no company is testing ALL of them. Similarly, companies may agonize over testing ie6 vs ie7 vs i8 vs i9 is 2017 but none that I can imagine are still testing IE3 or early netscape versions.
So at the end of the day it comes down to value. If you have 23,000 customers and $45.6 million in revenue per year and you can see that only 0.00023% of customers are using a version, that means only $105 of revenue and 1 customer is at stake. Probably not a high priority compared to a similar sized change that will raise revenue by 0.04% = $18,240