I think its because of inexperience. When these debut albums come out and they tour for it and stuff, they learn new things instrumentally and vocally and they apply that to their next album which will most likely have a bigger budget and a better producer and sound engineer. Lostprophets' first album, Fake Sound of Progress, was an amazing album but the sound quality is sketchy and most people thought it was shit. But when they released Start Something they had 3 years between them and in that time they got a major label contract (bigger budgets) they grew as musicians, leaving behind the "rap rock" sound behind, thus making a great post-numetal album that appealed to the mainstream market.
My favourite album is Three Cheers though tbh, I can't wait for the new one, and I wouldn't have a clue with books...