The core is a few microns narrower on one of these — and that single dimension is the reason one runs 40 km on a laser and the other tops out around 550 m on a cheap LED.
Order the wrong fiber patch cord and a link either won't come up at all or will train at a fraction of its rated speed — which makes it tempting to treat multi-mode as simply "the cheaper, worse fiber" and single-mode as the safe default everywhere. That gets the economics backwards. The two fiber types exist because of one physical difference — how wide the glass core is — and that width determines how many paths light can take through it. Multi-mode isn't an inferior compromise; it's the cost-optimized choice for exactly the distances most data-center and campus links actually need.