Think of it this way:
How is 172.16.10.6 /30 in the 172.16.10.4 /30 subnet?
172.16.10.4 /30
172.16.10.5 /30
172.16.10.6 /30 <---
172.16.10.7 /30
You've done your subnetting in your question. The last nibble "6" is comprised of bits 60 - 63 in an IPv6 address. A mask of /62 leaves the last 2 bits (62, 63) for subnet. So:
nibble = bits 60, 61 | bits 62, 63
4 = 01 | 00
5 = 01 | 01
6 = 01 | 10 <---
7 = 01 | 11
Make sense?