As per the documentation:

Note that it's important to always use the same unit for defining your breakpoints or the generated utilities may be sorted in an unexpected order, causing breakpoint classes to override each other in unexpected ways.

Tailwind uses rem for the default breakpoints, so if you are adding additional breakpoints to the defaults, make sure you use rem as well.

If you really want to use px for your breakpoints, then you'd want to redefine all the breakpoints as px:

@import "tailwindcss";
@theme {
  --breakpoint-sm: 640px;
  --breakpoint-md: 768px;
  --breakpoint-lg: 992px;
  … 
}

The confusing part is that 400px and 992px are consistent with each other, but they are not consistent with Tailwind’s default breakpoints, which are rem based.

So if you only define:

@theme {
  --breakpoint-md: 400px;
  --breakpoint-lg: 992px;
}

you can still run into ordering problems against the remaining default rem breakpoints/utilities. Tailwind sorts generated responsive rules by the breakpoint values, but mixed units cannot always be ordered the way humans expect after conversion.

The two safe options are:

@theme {
  --breakpoint-md: 25rem;
  --breakpoint-lg: 62rem;
}

or override all breakpoints using the same unit, so there is no px/rem mix in the generated responsive utilities.

So yes, 992px and 62rem are the same at the default root font size, but for Tailwind’s CSS generation order the recommendation is still: keep all breakpoint tokens in the same unit, preferably rem if you are extending the defaults.