How Ned & Robert became wards of Jon Arryn

[zionius]

Most wards are fostered inside their own kingdom: Jaime, Quenty, Brandon, Doran, Harrold Hardyng, Tristifer Botley... Any exceptions must have good reasons: Theon & Myrcella as hostages, Big & little Walder as tokens of alliance, Merrett/Sandor/Cynthea/Red Walder Frey as too many Freys for the River lords to foster them all.

And for Petyr...Well, an SSM on that: 

Quote

Petyr's father and Hoster met up during the War of the Ninepenny Kings and became friends. Apparently that was a time when a lot of people from all over the realm forged friendships.

So what about Ned & Robert? Their fosterage are rather strange. Would you give the heir to House Baratheon to someone you hardly know, simply because of your ambition or maester's scheme(as suggested by Lady Dustin)? 

The puzzle is solved in A World of Ice and Fire App, which states under Jon Arryn:

Quote

He fought for King Jaehaerys II in the War of the Ninepenny Kings, and, as a result of the alliances made in that war, came to foster Eddard Stark and Robert Baratheon.

As for the timeline, the War ended in 260 AC, Robert was born in 262 AC, and Ned in 263 AC. Plus, the Vale lies between the North and Stormland. So wards at the Eyrie are easier to return home from time to time as stated in this SSM. Everything just fits!

So when GRRM said "that was a time when a lot of people from all over the realm forged friendships", not only did he mean Tywin, Aerys and Steffon, but also Jon Arryn, Steffon and Rickard.