The use of “an” with “history” is likely a holdover of its French origins where the h isn’t pronounced or from English dialects where the h isn’t pronounced. It

The use of “an” with “history” is likely a holdover of its French origins where the h isn’t pronounced or from English dialects where the h isn’t pronounced. It’s essentially a learned behavior at this point. If it was used solely because of ease of pronunciation we’d expect also to see “an history”, “an high five”, “an hit record”. But, in any case, for those who pronounce the h, “a historical” is easier to say. In the case of lawyers I’d suspect tradition first and fear of typos reversing meaning (“ahistorical”) second.