What is the difference between "Total Military Strength" and "Effective Military Strength"?

It is simple as this:

The Total Military Strength is the numerical total of all of your War assets.

The Effective Military Strength is that total, multiplied by your Galactic Readiness.

Judging from your numbers, your Galactic Readiness is sitting at 50%

The Effective Military Strength determines your ending.

Total*Readiness=Effective

Total Military Strength is the total sum of all War Resources you've acquired. This is multiplied by the Readiness Percentage from the Galaxy at War mechanic, which can be increased by engaging in multiplayer, and playing the Mass Effect:Infiltrator iOS/Android game.

Effective Military Strength is what determines the ending. This means that if you enter the final mission with a total Military Strength of below 10,000, you can manipulate the ending via Galaxy at War to see a variety of endings. The exact amount of manipulation possible will vary based upon your total, but it seems that to see the broadest possible range of endings on one playthrough, you would want your total assets to be somewhere between 4,000 and 5,000 - note that this does preclude the 'best' ending however.