Crown growth and graft location.

Good questions mate!

Generally growth in the crown region is slower and the answer as to why, - Possibly blood supply in that area Its very common for crowns to take much longer to reach their fully grown out stage.

1's are used on the hairline to recreate the illusion that mother nature intended - soft and featherd. 2's 3's and 4's (Multi's) are used behind the hairline to allow the HT'd area to achieve density, coverage and volume and are planted in such a way to aid each other in the illusion of density and coverage.

Multi's along the hairline do not look natural (I have had personal experience with them there) But when placed behind the hairline, they help aid coverage and can't be see as multis.

Good artistry makes all the difference between an average HT and an excellent HT. Great docs can make a few grafts go a very long way and great artistry design and placement allow these multi's to create excellent coverage in areas that are not heavily scrutinized unlike the hairline.

Hope this makes sense mate.
Spex