You might say that a regular electric heater is 100% efficient, converting every unit of electricity into exactly one unit of equivalent heat.

Crypto mining hardware is >100% efficient, as it's exactly as efficient as an electric heater, but also generates revenue. Over time, others make more efficient crypto mining hardware, reducing the yield from yours. This causes your "efficiency" to gradually reduce down to 100%, but no lower.

However, nowadays we have heat pumps, which are also more efficient than our 100% benchmark, and produce additional useful heat.