Two headphones can list the same "105 dB" and be wildly different in how hard they are to drive, because manufacturers quote sensitivity in two incompatible units and rarely label which one. Paste in whatever the spec sheet says, add the impedance, and get both.
One volt into 600Ω delivers 1.7 mW. The same volt into 16Ω delivers 62 mW — about 36 times more power. So for a high-impedance headphone the two numbers sit fairly close together, while for a low-impedance IEM they can be more than 15 dB apart. That is why the unit confusion does the most damage exactly where people are most likely to get burned: cheap, sensitive IEMs.