MPs?

Just a few moments ago I happened to see the descriptors following the names of various national politicians.

After the names of those individuals who were members of the House of Commons the letters 'MP' appeared. After the Senator's name it said 'Senator.'

In many ways this seems somewhat strange because both Senators and members of the House of Commons are members of Parliament. How is it that common usage has developed in such a way that only members of the House of Commons are referred to in this way?

Presumably this has something to do with the corresponding usage practices that have developed in the UK, though it might be interesting to know if this has always been the case. I wonder if there has been a time when Lords were known as MPs?

Surely all of you are equally as concerned with this question and will likely lose some amount of sleep in the coming days, at least on until some sort of resolution has been reached.