There's actually a reason why the zombies in the mansion are more rotten looking in RE1 though: they had been dead for around 2 months (since May 1998). In RE2 the outbreak takes place over the course of a week or less, so it makes sense that the zombies are fresher.
My favorite lore fact is how the T-Virus causes insane itching to the feet, leaving victims to remove their shoes before fully succumbing to infection.
It's a bit more complicated than just time.
Zombies in general are still
living humans, they're just decayed and no longer need their vital organs. The Progenitor Virus already causes necrosis, you can see "Progenitor Zombies" in BIO5. However, the t-Virus causes advanced necrosis due to the leech genes used to create it, as leeches are capable of surviving in the absence of oxygen by feeding on the lysate of dead cells.
Essentially, Zombies in this series literally eat themselves inside out. This was CAPCOM's solution to Day of the Dead's lack of exposition on how Zombies work aside from "look at how they don't need their organs." Being able to survive without oxygen means the heart and lungs are unnecessary, while the spinal cord, mesencephalon (motor control, vision, hearing), thalamus (smell, motor functions) and limbic system (anger, hunger) developed abnormally and remained active. The rest of the brain decayed. So the only way to truly kill a Zombie is to destroy those portions of the brain. It's also stated that, since it's believed an animals appetite is controlled by the hypothalamus, its hypothesized that even a Zombie wont attack people if the hypothalamus is destroyed.
Originally, the advanced decomposition of BIO Zombies was due to a decrease in metabolism. However, when leech genes were introduced, they changed it so that Zombies had an
increased metabolism, one of the only genuine retcons in the series.