Unsure about accuracy scores after short games

So I just crossed the 1000 mark in rapid, and with a blunder my opponents are somehow still making. Its right after the opening "Italian game Kight attack Lolli defense" and basically just consists of them not noticing a knight is now defended by my bishop because I played d4 and not d3. so its a series of 6 book moves, followed by the best engine move (taking a hanging queen), but I only get 72% accuracy somehow, does anyone know why this is? or have any more examples if this fun blunder at higher levels 

The key in this deals with Book moves.  The engine does not grade all Book moves the same.  Behind the scenes, the engine is still scoring each Book move individually.

Thanks for the response, that does make sense, for some reason I thought the engine assumed all book moves were perfect