What makes a CPU work hotter under the same conditions?

I have a couple of Lenovo x201 I found in a dumpster a few years ago. Both have the exact same specs, and both required a new LCD screens so I got two identical LCD modules for them.

Usually I only use one of them, but lately it had some motherboard issues so I had to use the other. I've moved my hard drive from the usual laptop to the other one, and continued to use my laptop as if nothing changed.

After about 30 minutes using the other laptop, I noticed that it's blazing hot, and shortly after, it shutdown due to over heat. I figured the cooling system was busted, or that the intake/exhaust on the case was clogged, so I simply took the motherboard from the overheating laptop, and put it into the case of what was my regular laptop (which never overheat in the past).
It still shutdown due to overheat.

I've since fixed the broken motherboard (busted SMD fuse), and it's working like a charm - no heating issues whatsoever.

Why did the same CPU model, in the same setup and same usage work hotter?

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