Converting between Zara and H&M trips people up because the two brands are cut differently. Zara runs small with a slim cut, while H&M runs slightly small. H&M generally runs larger than Zara, so the same body usually takes a smaller size label at H&M. The converter above corrects for this automatically by reading the body-measurement range behind each size rather than matching the letter on the tag.
As a quick guide: a size S in Zara converts to about a S in H&M, a M lands around a M, and a L is roughly a L. When your measurement falls near a boundary the tool flags it as a between-sizes case — worth checking the garment's own measurements before you order.