What i don't get is why it refers to sphere as apposed to the earth?
Because as a "first approximation" (in other words "roughly speaking") the Earth is a sphere.
and also why you constantly change direction as a result of convergence?
Your're not really changing direction, you were right first time when you said you have " a constantly changing track direction". Because the meridians/lines of longitude
converge towards the poles they are not parallel ( again because the Earth is a sphere) the angle a great circle makes to the lines of longitude ( your track) changes as you progress along the great circle.
To visualise this take a globe, try stretching an elastic band in a straight line between say LAX and London and see how the angle between the band and the meridians, your track, changes as you progress along the "route". If you get it right you "leave" LAX tracking to the northeast, somewhere along the route you will be tracking east, and near London your track will be southeasterly - but your great circle has always been a straight line.
( and before someone points it out there's at least one exception to the track changing direction rule - the equator).