Two bottles of "magnesium 400mg" can deliver very different amounts of actual magnesium. Here's the short version of why, and what to check.
Magnesium, calcium, and iron are rarely sold as the pure element: they're bound to another compound (magnesium glycinate, magnesium oxide, ferrous sulfate) because the pure element isn't stable or absorbable on its own. A label listing "400mg magnesium glycinate" is describing the weight of the whole compound, not necessarily the weight of elemental magnesium inside it. Magnesium oxide, for example, is a much higher percentage filler by weight than magnesium glycinate. Two labels showing the same number can mean meaningfully different actual doses. Reputable labels increasingly call out "elemental" amounts directly; if a label doesn't, that's worth noticing.
The compound a nutrient is bound to also affects how well it's absorbed and how it's tolerated: magnesium citrate and magnesium glycinate are absorbed differently and have different gastrointestinal effects at the same nominal dose, for instance. The dose number alone doesn't capture that.
When comparing two products for the same nutrient: check the specific compound form, not just the headline number, and check whether the label states an elemental amount. If it doesn't say and you can't tell, that itself is useful information about how carefully the label was put together.