The "catching up" timer can of course go up, if the other players run at e.g. 25 fps and you can only simulate at 10 fps. That means the frame difference between you and the other players keeps going up = time goes up (not to mention the simpler case where the frame difference is constant but your simulation gets slower because more and more things are happening). Compare to a situation where your job is to take apples out of a box (=simulate the game), but others keep putting in more apples than you can take out. So the amount of apples in the box (=frames to simulate) will go up.
As for the ETA timer,
Brackman already told you why it happens. If you spend 20 metal per second at first and then only 2 metal per second later on, the required build time will go from e.g. 10 seconds to 100 seconds. It can't predict the future (because that includes your decisions), so it doesn't know while spending 20 metal per second that it will spend less in the future.
In an example, if you have 20 people building a house and then decide to take away 18 of them, it'll of course suddenly take longer to build the house. Nobody could've known at the start that you'd do that, and the two people left will of course tell you that it takes longer now.