You’ve ignored the fact that the process of spilling and soaking into clothing would have further cooled the coffee. Suppose it was served at 190 degrees and when she spilled it it was at 180 degrees. The average temperature of the coffee incident on her skin might have been 160 degrees.
Now suppose it was served at 170 degrees and spilled at 160 degrees. The average temp incident on her skin might have been 140 degrees.
