When I add a structure to the queue, it affects imediatelly my base, why?

It gives you a estimative, what you will have after it is completed.

It is necessary to the system enable the queue links to the other structures that require the benefits of the previous one, like more power or population

If you have a few items on queue, and you add a Metal Refinery that increase your construction points, it may show a faster time than before to the previous queue items, it will be improved later