What if 273 out of the 336 times, the price had also increased too, between the retarget periods?
If every time the price went up the difficulty followed, then yes, that would be correct. But that's simply not what happens. Sometimes the price goes down and difficulty goes up. Sometimes the price goes up and the difficulty goes down.
What is the common assumption one can make if every other factor remained the same and only price was variable?
That's the whole point - there are too many other variables to say that price accurately determines hashrate. The availability of electricity, the price of that electricity, local disasters/storms/floods/blackouts/etc., the availability/production/cost of new ASICs, the availability of new locations to host mining hardware, the profitability of mining other coins, local laws/regulations/taxes, the list is enormous. The interplay between all these things is far too complex to be able to say "x goes up, so therefore y goes up".
As I said above, if the price fell to $10, then that would be a big enough change to overcome all these things and the hashrate would tank. And if the price rocketed to $1,000,000, then again, that would be a big enough change to overcome all these things except the availability of new ASICs. But at current prices, the price does not exert enough of an influence to cause significant changes in the hashrate, as we have just seen by the price falling by 50% but the hashrate increasing by 33%.