After hundreds of years of Polish-Lithuanian Commonwealth Poles were emigrating in the east and this fact didn't make Lithuanians of them.
I suspect you exaggerate the number of such emigrants. They were but not as much as people who could speak Polish in 18 century.
The surnames should be also mentioned. They are in many cases of Ruthenian origin and couldn't appear in Poland. People with some surnames didn't live abroad Litwa for centuries.
Only the examples of three ancestors' families I managed to learn the past untill 17 century: Lukianski (first L is Polish I can't type), Onichimowski and Hryncewicz.
They all (ancestors) lived only in Litwa.
Lukianski isn't a Polish surname I was told. In Poland it could be something as Luczanski.
Onichimowski as I suppose is Polonized version of Anisimowicz (from Ruthenian name of Anysim). As far as I know The place the family appeared was in Wolkowysk (to South from Grodno). I took in Internet telephones of People with such surname in Poland. Two persons are Warszawa residents. Others live in ex German territories that means they all are from "Kresy"
Hryncewicz as I was told by experts in Polish and Bielorussian is also of Ruthenian origin.
I found some other surnames of ancestors I didn't manage to discover the past.
Stryewski - other people with such surname I found have origin only in Oszmjany powiat Golszany gmin my grand-grand-grandmother with that surname was resident.
Szymont - the end of that surname (mont) shows its Baltic origin not Polish one.
There were some other surnames I don't know the origin: Jacunski, Szyszlo. If they are even 100 per cent Polish that is the minority.
You see that is a common family from "Kresy".