You must be getting faster bursts on smaller downloads, it's causing the system to think it needs to test your connection longer than necessary. I see this myself time to time on LTE and will be improving the algorithm in the future. As well as offer a settings page where you'll have the ability to input a max test size to give users greater flexibility.
Try going to the Download Speed Test, then select your test size manually from the drop down. Sizes greater than 12MB are set to not forward, so if you select 13MB it will produce a result even if it thinks you should test with a larger size.
Look down in more sizes to get right at 13MB.
You actually should, at your speed, be able to select 6MB from the list and it shouldn't forward you. But if that doesn't work keep in mind the tip on sizes 13MB and larger.
Thank you for reporting this, I'll use your results to simulate the past tests you've ran to help make it better for you in the future.
