The phrase "actually narrow" is correct and usable in written English.
It can be used to emphasize the true or real state of something being narrow, often in contrast to a previous assumption or belief.
Example: "While the river appears wide from a distance, it is actually narrow when you get closer."
Alternatives: "truly narrow" or "genuinely narrow".
There is some evidence that achievement gaps between high- and low-income students actually narrow during the nine-month school year, but they widen again in the summer months.
Our target to turn this around is hugely ambitious: it means making poor children so much better off that they actually narrow the gap with average families, whose prosperity has increased rapidly in the last few years".
"If you just look through the lens of this referendum that is behind us, that will actually narrow the way in which we look at ourselves," he told BBC Radio 4's Today programme.
Search committees are often just trying to get as much detail about candidates as they can get, but in some cases their goal is to actually narrow the pool.
The changes in the budget actually narrow the tax base, but changes to taxes like the GST – because of their regressive nature – would necessitate increased government spending, in effect shifting our economy to more of a European-style one where tax revenue and government expenditure overall is much higher.
Engineering degrees can be very technical and can actually narrow one's horizons.
But going above that, it actually narrows even more, and it is really awful.
This is a pleasing illusion: the island actually narrows at this point.
From the end of World War II until the late 1960's, it actually narrowed a bit.
The patent is widely known as the "pinch to zoom" patent, but the software is actually narrower in scope.
The growing taste for watching soloists play from memory has actually narrowed the breadth of the repertoire.
