Exactly what you're saying. Even outside of the whole "narrow focus" thing -> you're inherently trying to fit the jigsaw pieces (facts) within your own mould/framework, while maybe leaving out important pieces.
On one hand, can't KNOW what pieces are important (slavery)
But he also actively encourages to leave out facts so we can fit things to mould our view / aim (find example?)
Consequently, Carr advocates for narrow, biased interpretations of history which, while they may bring to light relevant information for the present, do not bring to light everything.
Even appears to prize people who have done this, sayign great history tells us as much about a historian's society as the one under study.
OVERALL...
while I agree with Carr's definition of history as a progressive science and continual conversation between past and present, his notion that the historian must give priority to some facts over others is misleading.