The 08:05 service originates from Uckfield and runs semi fast upto Oxted then fast to East Croydon, so you dont pick up passengers from the intermediate stations like Sanderstead and Upper Warlingham.
Secondly its timing has changed since the timetable change in May, it used to be the 06:58 or 06:57 departure from Uckfield which has moved forward to 07:08, as a result a large majority of the passengers switched to catching the earlier train (06:34) so they would arrive into London earlier, hence more space.
Until the timetable change it used to be the busiest service up that line which is why its formed of 8 coaches (2x4 171 units). The 06:34 service which is now packed has had to have an extra 2 car unit added to make it a 6 car service to accommodate the extra passengers who moaned like hell.
Unfortunately the downside is that the first off peak service (08:34) leaving Uckfield is only a 2 car service meaning its pretty much packed out halfway up the line.
Hope this explains the reasoning why its quieter