Are A055999 and A074171 somehow the same?

[Jud McCranie]

At 03:41 PM 10/4/2004, Alonso Del Arte wrote:

A055999 is a(n)=n*(n+7)/2, which works out to be ((n^2 - n)/2) - 3n
>with a different offset. A074171 is Start with 1, add the next number
>if one gets a prime then subtract the next number else add the next.

I'm unable to understand how A74171 is generated.  For instance, with 
a(4)=4, add the next number, 5, gives 9, composite, so 4+5 = 9 is the next 
term.  Then a(5)=9, add the  next number (10?), gives 19, prime, so 
subtract 10 from 9?  What am I doing wrong?