How that H sneaked in there is a mystery to me.
No mystery at all. Before the 20th century, this was the standard orthography; in 1901 ⚠️ ↗ the spelling th was simplified to t (except in words of Greek origin). To be honest, the th in certain English words like Thames (Latin Tamesis, Old English Temes) or Anthony (L. Antonius) is much more mysterious.
Surf safe;)
NFiH