Lenses will display the results of scopes on a first come, first served basis
When a search term is entered, each scope performs their search gets the results and then sends the results to the lens to be displayed. The lens displays the results in the order in which they are delivered to it. So, scopes which finish their searches quicker get their results displayed first.
A number of factors affect the search speed of a scope. Some websites may be slower at returning results than others, for example. In this case, Grooveshark's search is presumably finished before Spotify's, and thus, its results are delivered to the lens first.
It would be possible for a scope to have a delay built into it in order to de-emphasise the scope's results, but it is difficult to see how a scope could perform searches quicker, especially if it is reliant on a website delivering it results to display.