The Mishna in Rosh Hashana (16a) states that there are 4 times the world is judged. On Pesach on תבואה, Shavuos on fruits, Rosh Hashana man is judged, and on Succos for water.
The Ran asks that the din of a person on Rosh Hashana would seem to include all the others (tevua, water, and fruits) so what is the din on Pesach, Shavuos, and Succos on these things? Weren't these already decided on Rosh Hashana? The Ran answers that the din on Rosh Hashana is for the individual while the din on Pesach etc. is for the
tzibbur as a whole (maybe each country). In other words, on Rosh Hashana each person is judged individually what is going to be their fate, on Pesach the country as a whole is judged on their תבואה.
The Turei Even is bothered by a number of questions on the Ran.
1. He points out that the gemara says that if your wheat is damaged by something, before Pesach it is last years din, after Pesach it is this years din. It seems from the gemara that this (the din of tevua on Pesach) is relating to an individual person not the tzibbur.
2. If on Rosh Hashana every person has a din on how much tevua he will get then what is left for the next Pesach, from Pesach until Rosh Hashana was decided on each individual how much they will get (and therefore it was decided how much the tzibbur as a whole will get).
To answer these questions the Turei Even says that the gezera on Rosh Hashana for individuals is not a fixed amount but rather a percentage. Reuven will get x percent of his country's wheat. How much that will be will only be decided on Pesach when the din on wheat for that country is done.
Abaye makes a seemingly difficult statement here (16a). He says that if you see that your winter תבואה is succeeding it means that last Pesach the din was favorable. Therefore, he says you should quickly plant so that you can get another crop in before Pesach and benefit from the favorable din on the previous Pesach. The obvious question is wasn't your allocation fixed on Rosh Hashana? It sounds like Abaye is saying that you can somehow get more then was allocated to you on Rosh Hashana.
According to the Turei Even we can say that while the percentage is set the distribution is not. Abaye is saying to get more before Pesach so that in real terms you will end up with more.
Here is a simple example with numbers. On the Pesach before RH it is nigzar that your country will get 1,000,000 bushels of wheat. On RH it is nigzar that you will get 1% of that. The next Pesach it is nigzar that your country will get only 500,000 bushels of wheat. If you get 50% of your allotment before Pesach and 50% after Pesach you will end up with 0.5% of 1,000,000 which is 5000 and and 0.5% of 500,000 which is 2,500 for a total of 7500. What Abaye is suggesting is that you try to get 70% of your allotment before Pesach and only 30% after. If you do that you end up with 0.7% of 1,000,000 which is 7000 and 0.3% of 500,000 which is 1500 for a grand total of 8500. In other words even though you got the same 1% percent of your country's total in real terms you ended up with an extra 1000 bushels.
Many times, the word תבואה represents not just grain, but parnasa as a whole. If that is the case here then it turns out that a person's income is not fixed in absolute terms on RH. Rather, it is set as a percentage of the total income of the country. Therefore a person could increase their income by doing what Abaye suggested, shifting the distribution around. This could mean that there is justification for a person working harder to get a bonus in the winter, his hishtadlus really does increase his net income and this is not in contradiction to the fact that a person's income is fixed on RH because what is fixed is the percentage not the actual amount.