There could be several reasons. I fly a bit lower than jets so their view may be better. Sometimes weather ( ie rain ) is very heavy and the return from radar is just all green with slight yellow and red. As you pass thru the rain the radar can see further. In troughy weather particularly it may not be easy to "see" very far ahead at all.
Another possibility is that the radar is covered in rain as described above and then as you come out of heavy rain you visually see a huge cell a few miles ahead that radar missed and need immediate diversions.
I have also been flying towards benign looking weather according to the radar and visually looks OK and then a huge lightening strike will appear directly ahead and confirm not all is well.
One further point is this type of weather pattern produces all over mess and you need to get up close sometimes to see the best way thru. In frontal weather its much more obvious and long distant adjustments can be made.
There is only some much radar can do using gain, tilt, etc particularly in a trough system like today. We sometimes have to change close to the mark. Most pilots flying jets and regional turbo props are experienced and will alter course earlier than later if the facts present themselves. Hope this helps a little