It didn't mean anything.
The reality in 1944 is that they were dividing Southeastern Europe into British and Soviet spheres of influence. A country, if it was not neutral or jointly occupied (i.e., 50-50), could only be part of one sphere or the other, not both. At least, not in any tenable way for any length of time. In practical terms, therefore, each Balkan country was effectively allocated to whichever side had a bigger "percentage" in the agreement. This is apparent from the fact that neither one attempted to discuss how this percentage could be measured or the allocation implemented:
This entire agreement was made in rather nonchalantly at the dinner table . . . the modalities of measurement never were discussed.
Johnson, Lonnie. Central Europe: Enemies, Neighbors, Friends. Oxford University Press, 1996.
A curious lapse had they intended the numbers to be meaningful. As experienced national leaders, Churchill and Stalin were not naive enough for this to be an accident. In fact, it's evident from the way Churchill referred to the agreement that he considered the the percentages to essentially be only for show. As he remarked in 1956:
Stalin never broke his word to me. We agreed on the Balkans. I said he could have Rumania and Bulgaria; and he said we could have Greece. He signed a slip of paper. And he never broke his word. We saved Greece that way. When we went in in 1944 Stalin didn't interfere. You Americans didn't help, you know.
Sulzberger, Cyrus Leo. The Last of the Giants. Macmillan, New York, 1970.
There is no trace, in Churchill's description of the deal, of Stalin promising him anything in Bulgaria, where they supposedly agreed on a 25% share for Britain, let alone a measly 10% in Romania. Likewise, Churchill indicates no Soviet reservations on Britain "having" Greece, where the nominal percentages were the inverse of Romania's (i.e., 90% to the UK and 10% to the USSR). Not only did Churchill leave the meeting believing he had secured for Britain a free hand in Greece, but the Foreign Office even felt empowered to request Soviet support for their Greek policies.
[A] clear if informal deal had been done on the poin that mattered most to Churchill: he had Stalin's consent to handle Greee as he saw fit . . . [Foreign Secretary] Eden told the Soviet Ambassador [that] as Greece was in the British theatre of command, "we felt entitled to ask for Soviet support for our policy there in return for the support we were giving to Soviet policy with regard to Romania" . . . The Soviet reply on 18 May was positive.
Carlton, David. Churchill and the Soviet Union. Manchester University Press, 2000.
This interpretation was evidently accepted by both sides. As Churchill noted, Stalin did actually stay out of Greece, allowing the communists there to falter and fail without Soviet support.