Yes, you're overlooking something. Look at the 61-digit prime, 9968667351197964859567689459993713773777797133337931717971339. 99% of all 61-digit primes are l

Yes, you're overlooking something. Look at the 61-digit prime, 9968667351197964859567689459993713773777797133337931717971339. 99% of all 61-digit primes are less than that prime. The 63-digit prime above it prepends a '2' and appends a '7'. If any of the 99% of 61-digit primes less than that prime prepend any digit between 3 and 9 while remaining bitruncatable, then they would be a bigger 63-digit prime. There's no guarantee that the biggest N+2 digit prime comes from the biggest N digit prime. It could come from the smallest N digit prime if that's the only N digit prime that becomes an N+2 digit prime by prepending a 9.