First off, this is a variety thing, not an error thing, right? so which topic should this be posted under in stead of error coins? Second, are these "small date" coins? and is the S MM on the right noticeably different? the top part of it seems smaller. are these small date with the one having the small s as well?
Does the mint mark location have anything to do with it? for example on the large date here: http://www.lincolncentresource.com/smalldates/1970Ssmalldate.html the MM is always way up in the date for the "large" date varieties - whereas on these coins and the "small" example there the S is lower ?
The mint marks were punched into each die by hand, so the locations will vary on both large and small dates from one die to another. Because the mint marks were punched by hand, this can also lead to different mint marks having a different appearance. The strength and angle of the punch varies.
Over here http://www.coincommunity.com/forum/topic.asp?ARCHIVE=true&TOPIC_ID=40869&whichpage=5 the make a big deal about the east west placement of the mark, large being W, small being east
Doesn't matter. We can tell by other, more consistent characteristics that both your coins are large dates.
