Because 'otto' is actually plural. 8 being greater than 1. It's basically when telling time it's nearly always le + number. One is different. See http://italian.about.com/cs/travel/ht/telltimeitalian.htm for details
Time in italian is usually told as "Sono le number" like "Sono le otto" similar to how we say "It is eight". Except in italian when using numbers (higher than 1) they use the plural "sono" instead of "è".
Sono le otto = it's eight
meno un quarto = less a quarter
Note: It's "quarto" (quarter or 4th) not quattro (4)