What it is, really, is lazy writing.
In ANH and ROTJ, we have Death Stars, weapons that can destroy planets. In order to trump that threat, instead of using imagination, some writers created weapons that could destroy solar systems, or whatever.
By the same token, we have lightsabres, a weapon that can (within common sense limits) penetrate anything. And again, lazily, writers trump that by introducing 'lightsabre proof' stuff. It is very lazy indeed; luckily this sot of thinking does not infest the films.
The forcefield seemed resistant rather than impenetrable, but again, that's only logical.
As a sabre has a physical presence it can penetrate a forcefield slowly like any physical object can, so literally spekaing it will never 'cut' through a forcefield at all. Though if you follow the logic that enough firepower will bring down a field then it might.