Someone typed politrikum. What did they hear?
heard as politrikum
spelled polytrichum
They heard the word accurately and spelled it phonetically. Every sound is right; the letters are polytrichum.
What the ear did
This is the fully phonetic version: i for the y of polys, k for the chi of trichos. Both substitutions are the ones English would make if the word had been coined here rather than borrowed. It was borrowed, so both are wrong.
P-O-L-Y-T-R-I-C-H-U-M
The full entry for polytrichum covers the name itself.