There is no meaningful average time, because every web page, every web site is different. A good session time on an information-rich blog would probably be much longer than the session time for a shopping site that advertises quick checkout, where people land, add something to their cart and are done in under 90 seconds.
It all depends on what the content is (and how much of it there is) on the "site content" pages versus what's on the "landing pages," what offer is being made, what you are asking your visitor to do next, what sorts of visitors are landing on those pages, etc. etc. etc.
Sometimes a short session duration is good: it indicates that people were able to easily find what they were looking for. This could mean they'd be more likely to come back to you the next time they need that same thing, because they know your site is quick and easy to use. On the other hand, a short session time could be bad if it means that people found your site impossible to use so they left quickly. Same thing with a long session time: it could mean they were really interested in what your pages had to say... or it could mean they were wandering around aimlessly trying to figure out how to accomplish their goal before finally leaving in frustration.
In other words, given the information you've provided:
- It is not possible to diagnose why there would be a difference in the session time; and
- It is not clear whether a long session time or a short session time is "better" for your site; and
- It is not clear whether the difference that exists even matters in the first place.
I'm inclined to think that you're fretting over something that doesn't really matter. It's very easy to get sidetracked on the web because there are so many metrics we can track — we forget sometimes that not all metrics are significant.
Is there a difference in conversion rate between the "site content" pages and the "landing" pages? Unless you're making money based on how long people linger on your pages, I'd be paying a lot more attention to conversion rates and revenue than I'd be worrying about session length.