Mohr

You should consider the possibility that the trainers might be improving the course through the changes they make. There can be many reasons for making changes: the material has become obsolete, the learners already know some of the material, the learners have a specific application that they want to study, the trainer doesn't think the material is sufficient or interesting enough, and so forth.

Good trainers are able to adapt courses to the needs of their audience. My recommendation would be to set up (and incentivize) a feedback loop that enables course developers to learn from the changes that trainers are making and the reasons those changes have been made. Developers might discover that the trainers are one of the best sources for identifying new training needs. Those kind of systems can turn companies’ training organizations from support to leadership.

Hartley

The problem you posed is one that definitely happens routinely.

The structure of the instructional materials and the qualification or train-the-trainer process that the instructors go through can have tremendous effects on the way the training is ultimately delivered in the field. As long as the core content is covered in the training, there can and probably should be some variation in the delivery. Trainers’ experiences can add richness to the content beyond the specific language contained in the instructional materials. 
 

But if instructors are affecting the quality of the instruction with their delivery, here are a few tips that might help:

Point out ideas or content that must be delivered verbatim with highlighting, bold font, underlining or a different typeface. Train the instructors on this point. Describe how the flagging works in the preparation pages of the instructional materials. Observe new trainers and make sure they follow the instructions.

Have a strict train-the-trainer program. Don't let everyone who wants to be an instructor become one. Select people who are willing to play by the rules established. Reward and reinforce those who provide the best training experience.

Weed out those people who cannot play by the rules.

Consider e-learning. If you are seeking to have a completely consistent delivery across your audiences, I highly recommend that you consider technology-enabled learning interventions. They can help ensure that the same content is delivered to everyone and, when designed appropriately, can save time, money, and opportunity costs associated with classroom-based learning.