Welcome gerry5,
Middle variance is how much the speed varied in percentage. Only the middle of the TiP is considered. So from 10% to 90% of the progress percentage of the test... the high and low readings in that range are used to determine middle variance.
You want a low number, means less flucutation. BUT sometimes clients that have bottlenecks are present can have almost no variance... so sometimes under 5% can be a clue to something else.
Mouse over some of the words throughout the site and tooltips will popup explaining things. Just a heads up, many people may not notice that.
Happy Testing!