Very true - but I don't think that's what we're seeing in the US polls.
If it's the same polling company comparing their one weekly poll, then all those X is +1 and Y is -1, don't prove anything on their own.
However, where you have multiple polls combined, showing a trend over time, then statistically that's the margin of error being all but ironed out. Any poll watcher who sees a poll that's out of sync, or shows a new direction, should always wait for further evidence.
In the US, there has definitely been movement towards Trump - and that's showing in multiple state level polls, where the majority of the close races have flipped from Harris to Trump. Some of those changes will be well within the margin of error, but put together, it's a real trend.
Now, as
@SWP's back has pointed out, there are problems with polling beyond the margin of error, which will skew all polling. I'm not sure I agree that it's definitely giving them a pro-Trump bias, as there are reasons why Trump supporters may also be under-polled. These effects also change between elections, with the polling companies having to be careful not to over-correct for the bias they showed in the previous campaign.