Why are some sites more prone to fluctuation than others?

[IPAlarms]

I am doing a link building program on two sites for a customer both targeting the same keywords. Site 1 is on page 1 for the main keywords and alternates between 5th and 6th. If we don't build links for a month or two, it drops to 6th. When we start link building again, it goes back up to 5th. Very predictable.

We use the exact same link building techniques for site 2, but the highest we can get the site is 7th on page 1. Within a couple of weeks of stopping the link building, site 2 drops onto page 2. During the link building process, it will often jump to the bottom of page 1 for a day or two and then straight back onto page 2. Very erratic.

Any idea what could cause such behaviour in Google when both sites are so similar?

Could it be the on-page SEO as opposed to link building?

 

[link8r]

It's a system, so you have to expect that not all parts are equal.

the other parts you're going to need to tell me about in order to help is

1. Age of domain A and B (i.e. length of continuously indexed pages)
2. Length of time A has held it's position

And the most critical:
3. The size of the index (i.e. how many pages in the search results for those keyphrases)

 

[MayaLocke]

Sites that are new or not yet "trusted" may find that they fluctuate in the search results more than established well known (trusted) sites.

 

[neojr]

I think many factors influence rankings besides backlinks, such as domain age, on-page optimization and frequency of content updating.