You have most likely stripped a sealer or a colourant unevenly, or removed years of soil from some areas and not others. Unsealed cement grout is porous and absorbs whatever it meets, so patchiness is nearly always about what is in the grout rather than the grout itself.
Work out which of the three it is
1. Uneven cleaning
The most likely. Grout in traffic lanes and around the vanity holds more soil than grout under the towel rail. Clean it thoroughly and the difference between "properly clean" and "mostly clean" shows up as patches. The fix is finishing the job: same cleaner, same contact time, same scrubbing, over the whole floor rather than the dirty-looking parts.
2. Sealer stripped in places
An acidic or strongly alkaline cleaner strips penetrating sealer. Where the sealer has gone, the grout absorbs water and looks darker; where it survives, water beads and it looks lighter. Splash water across the area — if some patches darken instantly and others bead, that is your answer, and the whole area needs resealing after it has dried out fully.
3. Colourant worn through
If the grout was ever colour-sealed, aggressive cleaning takes that coating off in patches and you are seeing original grout next to coated grout. Nothing brings that back evenly; it needs recolouring.
Efflorescence, which looks like the opposite problem
A chalky white haze that keeps returning after cleaning is not staining, it is mineral salts carried to the surface by moisture moving through the tile assembly. Wiping it off works briefly; it comes back while the moisture source remains. Persistent efflorescence in a shower is worth taking seriously — it means water is getting somewhere it should not and sitting there.
Getting to a uniform result
- Clean the entire area to the same standard, not just the parts that look bad.
- Rinse thoroughly and let it dry for at least 24 hours — grout looks different wet, and people judge the result too early and start over unnecessarily.
- Repair any cracked or missing grout before sealing anything.
- Seal the whole area in one go with a penetrating sealer, or colour-seal it if the colour is beyond saving.
The honest option people forget
Grout colourant is cheap, it is a weekend, and it makes a tired bathroom look genuinely renewed — uniform colour, sealed surface, no demolition. It only works on cement grout in sound condition, not on epoxy grout and not over movement cracks. Where it applies, it is the best value thing you can do to an old bathroom short of replacing it.