Sorry, I don't know why pollando is used. In fact, "polio" is sometimes used, as is "polujo". The following websites list all three:
(I may have gone a little overboard with the links.) "Pollando" does seem to be the most commonly used word of the three, though.
The word ending describes where the name was derived from.
en:Japan, Japanese (people) eo:Japanio, Japanoj
In this case, the country is named after the people. Japan- is the root. The country has the -io suffix, while the people have the simple noun -o suffix (pluralized here).
en:America (USA), American (people) eo:Usono, Usonanoj
In this case, the people are named after the country. The root is Uson-. The country has the simple noun -o suffix, while the people have the -ano suffix (pluralized here). You can further break this down: Usona (adjective form), Usonan (accusative adjective), Usonano (accusative adjective nominalized).
en:Poland, Poles/Polish (people) eo:Pollando, Poloj
In the third case, the name of the country is derived from the name of the people in a slightly different way. Pol- is the root. The people have the -o simple noun suffix. The country, however, has -land- in the middle (with the simple noun suffix -o). We know from this module that land- refers to country, so Pol-land-o may be read "country of the Poles".
From wikipedia article on Esperanto Phonology
Geminate consonants generally only occur in polymorphemic words, such as mal-longa "short", ek-kuŝi "to flop down", mis-skribi "to mis-write"; in ethnonyms such as finno "a Finn", gallo "a Gaul" (now more commonly gaŭlo); in proper names such as Ŝillero "Schiller", Buddo "Buddha" (now more commonly Budho); and in a handful of unstable borrowings such as matĉo "a sports match". In compounds of lexical words, Zamenhof separated identical consonants with an epenthetic vowel, as in vivovespero "the evening of life", never *vivvespero.