Most minerals aren't sold in their pure elemental form — they're bound to another compound to make them stable and absorbable. That compound has its own weight, and it's easy for a label to show the combined weight rather than the amount of the actual mineral inside it.
An example
Magnesium bisglycinate is magnesium bound to the amino acid glycine. A capsule might contain 500mg of magnesium bisglycinate — but only around 100–115mg of that is elemental magnesium; the rest is the glycine carrier. A label that states "500mg magnesium" without specifying the compound could mean either figure, and the difference matters if you're trying to reach a specific daily amount.
Why this isn't unique to magnesium
The same distinction applies to calcium (carbonate vs citrate), iron (bisglycinate vs sulphate), and many botanical extracts, where the total extract weight is shown rather than the specific concentration of the compound the extract is valued for.
What to look for
A label that states both figures — the compound and the elemental or active amount — is giving you the information needed to compare it properly against another product. If only one figure is shown, it's worth checking which one it is before assuming.