I haven't read any in-depth literature about Roman military organisation, but I was suddenly reminded about this.
http://www.roman-empire.net/army/army.html

The divisions were now of ten maniples. The figures are a bit unclear, but what is known is that the hastati maniple consisted of 120 men.

Subdivisions of all three ranks (hastati, principes, triarii) was one of ten maniples. A maniple is defined as consisting of 160 men. (Although the hastati are supposedly had 120 per maniple. The figures are confusing. I assume that the maniple was brought its full numbers by the addition of velites. i.e. 120 hastati + 40 velites = 160 men = 1 maniple)

It emerges that now it was the velites who were the more mobile troops who operated in the front of the army, stinging the enemy with their javelins, before retiring through the ranks of the hastati and principes
I don't know how much work the author of this site put into research, so I won't vouch for him. But if it's true it would explain the confusion.