Notice the difference in kB/s and mb/s the B has a big difference. 8mbps is roughly ~976 kB/s. So if you're pulling a steady 600kB/s you're in good shape. Once you figure in congestion, internet routing, and other devices on your network 600kB/s is what I'd expect.
It's all about the bits and bytes
Speedtests will show higher because your ISP is using "boost" which allows for small file downloads(like less then 5mb) and web browsing to get temporary speed increases to 15-20mb/s when the network load is low.