I'm not the best person even close to it to answer this question. But here goes .. lol .. the data is split into small packets of data for downlding. When you are sourcing from more than one source, then the next packet is taken from the next available source. Be that the 1st 2nd .... whichever it can get a hold of 1st.

The speed of your downld could be affected by a no. of factors. eg: your downld is being juggled in priority for a particular slot or tempo of downld bandwidth with other people who are using the other slots of a particular source.