Mixed member proportional systems always have two parts. The first part is the locally elected seats and the second part is the top-up selection process.
Simple MMP uses a compensatory top-up method. A compensatory top-up method means that the top-up selection process begins with consideration of the locally elected seats, so that the end net result is the most proportional result possible.
The opposite is a non-compensatory method, in which the two parts of MMP are independent events. Under this system, there are locally elected seats and then a completely independent method (often using a second vote on the ballot) to determine the top-up seats. At best, these methods produce a result which is somewhat proportional, since the local elections will produce disproportional results which are not compensated for during the top-up process.
Personally, I have never understood the need for having two different votes. I have seen the argument that I might want a certain candidate elected because I like them, but I want to support some other party. But .. I have trouble seeing any situation where this would be true. When I do my best to elect my local candidate, am I not also doing my best to elect that candidate’s party?
As Canada looks at new voting systems, there is a lot to be said for keeping the process simple. The system can be improved down the road once we have had some experience with it, so we do not need to implement complex processes now, when simple processes will accomplish the desired outcome.