Whenever a manuscript is copied, some mistakes will almost certainly be made. But manuscript transmission is not simply  a mechanical process of cumulative error. The scribe may notice errors in the exemplar before him and be able to correct them, even without recourse to another copy; so it is quite possible for his copy, the apograph, to be on balance more accurate than the exemplar. On the other hand, the number of errors corrected must always be less than the number made, and the overall trend will necessarily be towards a less correct text. Besides, some of the scribe’s ‘corrections’ may themselves be mistaken, and this kind of corruption is often more insidious than inadvertent miscopying, being less easily detected afterwards.