If this is electrum specific behavior then my apologies but the question I think is more general.  I have been in bitcoin for about a month and in that month my wallet has 5 transactions

Receive 3.7 BTC on Address A
Receive 3.3 BTC on Address B
Send 3.25 BTC so the inputs to the transaction were A&B, the outputs were my destination address and the rest to Address C (Change Address?)
Receive 6.3 BTC on Address D
Receive 0.4 BTC on Address E

The way I see it the sum total of my wallet exists in addresses "C", "D", and "E" correct?  What is odd though is that Electrum shows that Address "B" is still a valid receive address but not Address "A".  I would expect neither to be valid receive addresses at this point since they are both inputs to another transaction.

Does that make sense?  I'm trying to wrap my head around what would happen if I sent bitcoins to Address "B" and why its a valid address when "A" isn't.

Thanks