One of the main benefits of a sheepsfoot roller is its ability to knit the two soil layers or lifts together due to the knobs not only compacting but also intermixing and 'poking' the compacted upper and lower layers together. Just having two compacted layers are sometimes not enough especially if those layers/lifts are too thick and adequate downward pressure was not applied so it resulted in an 'inadequate seal' between the two layers or 'sheets' of dirt. Water, due to pressure due to depth, will 'want' to seep or move between the two 'compacted' sheet-like layerslifts esp if the layers have loose spots between the layers. Sheepfoot devices due to the inter-knitting of the layers tend to minimize this seepage between layers / sheets, providing the layers are not too thick thus proper knitting of layers does not occur. Contractors in a hurry will tend to make the soil layers of the core trench too thick and this can result in an inadequate seal between the two built-up layers of clay. As Tim says, proper soil moisture is very important to proper compaction.
A double barrel heavier sheepsfoot roller compacts better than a lighter weight single barrel roller. Single barrel rollers tend to bounce around and often do an inadequate job when dirt layers are uneven and 'bumpy'.
Comparison of ways to compact soil:
http://extension.missouri.edu/webster/pond-seal.aspx
