I believe the problem here is that you do not realize the difference between MB (MegaBytes) and Mb (Megabits). There are 8 bits in a byte, so if we take the 1.5Mb/s and divide by 8, we get .1875, which is roughly 187.5KB/s. So in reality your Task Manager only shows a descrepency of about 55.7KB/s. This most likely can be attributed to other traffic on your network, or even lag between Tixati's reading and task manager's readings. So this answers your question I think. As a side note, you can change how Tixati displays data rates and file amounts on the settings menu under "Output Formatting" in the "User Interface".
Also, You may want to set your incoming speed much lower, as in reality, you can only download at a max of about 1006KB/s. The auto limit option on the bandwidth page is usually a good choice if you don't know the best value to set it to.
For an article on data rates and the various meanings of these symbols, try reading: