specifications of a given device can change, and so would then e.g. the manual. To forestall complaints or even lawsuits, the manufacturer is obviously making sure that it is clearly understood that specifications can and do change over time (and that whatever information is now available may not be valid in the future).

I think they are saying that changes may have to be made for technical reasons to the thing being described, and that in that case the information wouldn't be entirely correct (until they get round to updating the information, of course...)

The statement is equivalent to saying "We reserve the right to make changes in the specification and/or the documentation."

The manufacturere is essentially preventing the consumer from holding them to the current product features or to the current product description. In particular they are allowing room for correcting mistakes or faults without being responsible for the effect of the changes on the consumer.