I can think of a few reasons why you might get different results. The plastic cover over the metal antenna might be thick enough to keep your skin away(inverse-

I can think of a few reasons why you might get different results. The plastic cover over the metal antenna might be thick enough to keep your skin away(inverse-square law). The version of WiFi has different frequencies(2.4 ghz vs 5 ghz). The Wi-Fi isn't actively transmitting at full power. Or you're simply not holding it there as long. I'm sure we could come up with a repeatable experiment for this that doesn't have any risk of burning.