Punished for parking within white lines

The parking bays in Valletta are colour coded in three colours, namely green (for residents, 24 hours a day), blue (for residents and visitors from 8 a.m. to 7 p.m., and for residents only from 7 p.m. to 8 a.m.) and white (for all, residents and visitors alike, 24 hours a day).

Last night, at 9.15 p.m. a warden issued my car, which is rented, with a contravention ticket stating that only residents are allowed to park in St Ursula Street after 7 p.m., even though the parking bays are marked in white.

The warden pointed out that recently a small nondescript sign had been put up at the entrance to St Ursula Street corner with St Christopher Street, indicating that only residents are allowed to park there after 7 p.m. Why this confusion?

Why is it that where all other parking bays in Valletta are clearly colour coded, a mere, nondescript, barely noticeable (only noticeable if pointed out to you after being issued with a ticket) sign is deemed to overrule the default regulations as indicated by the colour coding of parking bays in the rest of Valletta?