Rebbe Mechabed Es Ha’ashirim. The fact is they are doing more Mitzvos. As mentioned, the fact that they were given the opportunity for this Mitzva is Hashem’s Cheshbon.
The Maharal actually writes elsewhere that Tzedaka goes by the dollar amount, and it all adds up to one lump sum. With this he argues on the notion to split it up. He says there is no greater Mitzva by giving $50 twice than giving $100 once. Truth is, even those that say to split it up don’t necessarily mean that it is another Mitzva, rather it breaks your Teva another time.
Giving more is like doing another Mitzva. Giving when it is hard gets you more S’char within this Mitzva but it is not another Mitzva. In fact, the S’char of doing another Mitzva is more than the S’char of Tzar. This is seen from Shaluach Haken. It is considered an easy Mitzva while used as an example of where the S’char can be the greatest. This is how the Maharal explains that Mishna.
Oomis, we don’t know Hashem’s Cheshbonos in His detailed dealings with specific people. However, He gave us a Torah in which He describes His attitude and yardstick. Anna Nafshi K’savis Yehavis, I wrote myself down and gave it over — Rashei Teivos of Anochi as explained in Maseches Shabbos.