I've used TB's speed tester as a matter of routine for several years, occasionally comparing results with other sites and finding them generally fairly similar.
My ISP has just made major changes to its network. Since then TB has been showing my download speed as only around half what it used to be, with the values fluctuating rather more than usual. However, all the other sites I've looked at show the download speed to have increased, and that accords with what my throughput meter is telling me.
Any idea what has gone wrong?