Not 100% sure on this, but it may be due to ‘helo’ being more audibly distinct than ‘heli’ over a shitty garbled radio?
It could also be from different regional/national accents of English pronouncing helicopter as basically hee-lo-copter, sort of like how there are different pronunciations of Uranus or nuclear?
What’s really neat linguistically is that “helicopter” isn’t a compound of “heli” and “copter,” but rather “helico” (as in helix, helical) and “pter” (as in pterodactyl).
“Rebracketing” is when this happens (i.e., the split in the word is moved in colloquial language).