My current fixed wireless ISP Bluebox (Ireland) experiences this issue also, particularly when downloading from UK servers. If I download a large file, I typically get 1MB/s, but if I use a multi-threaded download manager, I can get up to 2MB/s and sometimes higher.
From what I heard, Speedtest does a multithread (without saying) and sure enough it reports 15.1Mb/s for me even when I rarely get this on a single transfer. With this site TestMy, I'm getting 8.1Mb/s in the standard download test and 13.2Mb/s in the multithread test using the UK server, which more closely matches my download experience.
In the evening, the variation is much greater for me where a multithreaded transfer can be 4 times faster than a single transfer. While this is fine for downloading, it sometimes affects HD playback on streaming services such as YouTube which stream with a single connection.
On my workplace DSL connection, I get 11.9Mbps with both the single and multithreaded tests.