Electrum uses hiearchial determistic(HD) seeds to generate addresses. Using the seed, the client can generate a master private key which can generate a practically unlimited number of addresses. The default behavior of Electrum is to generate addresses till they find an àddress with Bitcoins and generate an additional 10 addresses after that point. Judging by that, you likely had 16 transactions using your wallet.
I don't completely understand 
but thanks anyway. One more question, I exported those addresses, will it use these addresses only or it can generate new ones again and again, I want the same address.
You can use those addresses actually forever,as long as you are in possession of your seed and private keys.You do not have to worry about Electrum generate new addresses and you not need to export private keys for for each new address,all you need is your seed(secret words) and if you get in some problem with your wallet you can just download Electrum again and with your original seed restore completely your wallet with all addresses/balance/transactions.
There is also possibility that only with one public exposed private key from Elecrum HD you can jeopardize your wallet.If I remember well, there is a possibility to create seed using private key.