Dasaratha gave Kaikeyi half of payasam that remained, i.e. half of one fourth. Avasistardham, the one eighth that had remained he again gave to Sumitra. The word ardha here means exact half. Anucintya means considering that Sumitra deserved more than Kaikeyi. The reason for giving more payasa to Kausalya than to Sumitra and Kaikeyi is her seniority, one-fourth to Sumitra is her middle position, and one eighth to Kaikeyi even though she was the favourite queen was because of her junior position. One shows partiality to his favourite wife only in sexual enjoyment, not in dividing something related to dharma. So Bharata and Satrughna each obtained one eighth.
Distribution of payasam 1.16.27-31
Dasaratha gave Kaikeyi half of payasam that remained, i.e. half of one fourth. Avasistardham, the one eighth that had remained he again gave to Sumitra. The word ardha here means exact half. Anucintya means considering that Sumitra deserved more than Kaikeyi. The reason for giving more payasa to Kausalya than to Sumitra and Kaikeyi is her seniority, one-fourth to Sumitra is her middle position, and one eighth to Kaikeyi even though she was the favourite queen was because of her junior position. One shows partiality to his favourite wife only in sexual enjoyment, not in dividing something related to dharma. So Bharata and Satrughna each obtained one eighth.