I can't tell without an actual link, but suspect you are just zooming past the maximum resolution of that map. Look at the zoom level in the upper right corner of the screen. Level 17 is as high as that map goes. When you zoom that map to level 18, it just enlarges the level 17 map 2x; zoom to level 19 and it enlarges the level 17 map 4x. When you do that, it gets blurry. Compare level 18 parcels vs full res NJGIN aerials here
https://boydsmaps.com/sync/#18/39.697209/-74.526988/parcels07/0/0/njgin2007/18/0/0
Looks bad. Zoom back to level 17 and they are more similar. But the parcels map is still a little softer because it's been re-rendered/re-compressed with the blue lines on it.
https://boydsmaps.com/sync/#17/39.697209/-74.526988/parcels07/0/0/njgin2007/17/0/0
This is the parcels compared to my local version of the 2007 aerials. Those are also rendered at level 17 and should be pretty close in terms of quality.
https://boydsmaps.com/sync/#17/39.697209/-74.526988/parcels07/0/0/pines2007/17/0/0
So, I'm sure the next question is, "why don't you use higher resolution for the parcel map?" which is a fair question. But it's just a matter of storage cost (not to mention rendering time). The existing level 17 parcel map is about 32gb. Each zoom level requires about 4x the storage space of the previous one. Going to level 18 would add about 120gb of map tiles. Going all the way to level 19 (full resolution of the 2007 aerials) would add another 480gb. The parcel map would go from 32gb to 630gb! That would be over 20% of my total 3tb disk space which is just too much to devote to this one very specialized map that covers a tiny area. Not a good use of resources and money for me.
Will have to leave it to NJGIN and Google to offer level 19 imagery, my pockets aren't that full... in fact, they're pretty empty after paying the server lease!