Okay. I looked at one of the reports available. Here is what I can tell you.
Nibbler is only looking at 5 pages and gathers much of it's information from other sites. In fact, when you look at the various data blocks, you can see where the data comes from.
Also, please know that it is not always possible to get an accurate last modified date depending upon your website. Some CMS and Blogging software as well as others I am sure, block the last modification date based upon how they are designed. In fact, Google does not rely on the last modified date for the freshness date. That would be foolish. Google actually downloads each page and measures the difference as part of determining if a page has been updated. Nibbler cannot possibly know for sure that any page was or was not updated because it has nothing to compare the page to.
The Nibbler report is more detailed than any other I have seen and is user very friendly. No doubt. But let me warn you about all of these sites.
Take all of these types of sites with a healthy handful of salt!
For example, most if not all site use Alexa data which is extremely poor, wildly inaccurate, and down-right ridiculous as many of these sites can be. Another example, almost all back-link report sites have absolutely no clue about back links. Sounds odd, but it is true. This is an extraordinarily expensive process and with little exception, most of these sites are extremely poor and totally useless. As well, many of the keyword sites cannot tell you about the keywords that are returning users to your site. It is impossible for them to do this unless they are Google or Bing or even your own log file analysis tool. So instead, they list well behind the curve what keywords you seem to have that appear on the first several pages of the SERPs. Alexa's keywords is totally laughable. However, SEMRush is very accurate and timely.
The up-shot is this.
These sites have something to sell for quite a bit of money. And that is the point. They want your money. There is some valuable information that can be gleaned from these reports, but most, with very little exception, are junk. The intention of these reports is to get your attention and money and keep you spinning in your chair trying to chase an SEO dream. The entire industry is based upon fear, misdirection, misinformation, and just plain bull. You will find much Q&A on this site supporting this premise.
Most of what you need you can get for free from Google. The rest you can get from your log file or directly from the social sites you participate in. Stick with this and do not worry about what these sites say too much. Again, much of it is junk, but they can offer some insight that is useful. Use them as a jumping-off point and nothing more. Gather data directly from valuable original sources such as Googles Webmaster Tools and Google Analytics and you will be fine.