When I look at the Star Trek: The Next Generation episode "I, Borg", I notice that that Borg reacts differently than the Borg in Star Trek: Voyager.
Is there a specific reason for this?
A couple of points:
So a possible in-universe explanation could be:
It's likely the Borg in Voyager, 8 (or 5) years in the future, in their home quadrant and in vast quantities etc... would be somewhat different than a single Borg cube that Q introduced to Picard and the Enterprise D. Also, the Borg vessel in I, Borg had suffered significant damage and was cut off from the collective.
Essentially any differences that you see from the borg in the episode "i, Borg" stem from the fact that the borg on the enterprise is young, damaged, and most importantly cut off from the collective. The separation from the collective seems to affect each borg slightly differently as instead of relying on the hive mind, they are having to think for themselves, something they haven't done in a long time. the episode is also considering humanitarian rights of the borg, and whether giving the borg essentially a virus, which would infect all of the other borg, is acceptable in war or not. This is why the crew are interacting with the borg "Hugh" more so then usual. As to your final question about the looks and appearance, this is simply due to the time between shows, late 80s, to late 90s. Also for some in universe answer to this, it is somewhere between 5-10 years later, when voyager is encountering the borg, and at the borgs extreme rate of assimilation/ upgrading themselves its expected for them to look slightly different as newer and better tech becomes available to them.
Even upgrading themselves frequently, it is unlikely that Borg Cubes as distant from their home as the ones encountered by the Enterprise (7,000ly) would be as technologically advanced as the ones encountered by Voyager (75,000ly).
While the collective have the ability to communicate over such vast distances, it does not mean they have the materials to constantly upgrade to the technology level present in their home territory. So, a Borg Cube so far from home probably places a low priority on spending resources to upgrade the exoplating and accessories of their drones.