Having looked at a lot of the films at the LMA and then the actual registers, it's a combination of both, but more due to the state of the registers.
Many of the older registers are printed on parchment, which doesn't hold ink very well in some areas due to the nature of the skins, the other pre-printed registers are sometimes badly faded or damaged or both again.
As long as the register is not at risk (this is indicated in their parish register catalogues on the shelves at the LMA, you can ask to see the originals, but a member of staff has to counter-sign your request after showing them that the microfilm in question at the LMA (not the Ancestry image) is illegible.
Hope this helps.
Dawn