iCloud photo library has photo data, but is photo is set to "off"

Hello! iPhone 6s, recent upgrade (obviously) from 4s. After purchase, I transferred content from old phone to new using iTunes. Updated software to iOS9.1. Exploring settings and saw under Settings -> General -> Storage & iCloud Usage that my iCloud 'Available' was quite low. I clicked 'Manage Storage' and saw that 4.1GB displays under 'iCloud Photo Library'. I have not consciously enabled this and did not expect to see this.

I clicked 'iCloud Photo Library' and saw option to 'disable and delete'. When I selected this, I was told to download my data. To see what was available I followed instructions (received via email to go to Settings -> iCloud. When I navigated to this section, iCloud Drive is Off and Photos is Off. When I click on photos, all three options ('iCloud Photo Library', My Photo Stream' and 'iCloud Photo Sharing' are also Off. Just to see what would happen I tried to enable 'iCloud Photo Library', but this is not possible because photos on my device are > available capacity.

Key thing here is how under one setting option, 'iCloud Photo Library' has 4.1GB of data while viewing another setting and 'iCloud Photo Library' is Off. Has this discrepancy been encountered before, or is this in fact a valid state? To me this should not be possible.

I am also concerned I'll lose photo quality if my device believes photos are being synced to iCloud. Could this be happening?

I am also concerned that this was seemingly enabled without me realising that I had done so - assuming that this is an opt-in feature. Any way to see how & when this was enabled?