@JustWork thanks for indulging my grammar diversion 🤣
So, it’s historical (if I may use that word!).
In the 18th & 19th centuries, where an “h” wasn’t sounded or wasn’t stressed it was common to use an instead of a.
Take the word “history”. The stress is on the first syllable, so the h is clearly sounded, “A History of the Second World War”.
But if you say historical or historian, the stress is on the second syllable his-TOR-ian.
Even though the h is sounded, it was just the form at the time to say “an historian” (or other unstressed h word). You can hear why if you say “an history an historian” - the first feels clunky but the second doesn’t - even if it feels wrong to you, it won’t feel as clunky.
The practice has been dying out, but Fowler’s Modern English Usage (bit of a Bible on these things!) says both are acceptable and both are in use.