If i use a different bitcoin address for every transaction, it is still possible to correlate my addresses?

If i leave one address here in my signature (for tip, lets say) and this address only serves this purpose, i will never use it in any other transaction; it is possible to correlate it with other of my addresses?

I am reading that bitcoin is not so anonymous like people think it is, BUT what are the possible falws here? I know that every transaction is public, but if "they" cant correlate transactions/addresses with each other, they have nothing, right?

Explain like i am five please 

Thank you