Perhaps at the start of the game then you don't have bonuses to production/research? This is because of rounding you get lesser sum of spending to miliatry,social and research and so less net income with balanced slider settings compared to 100% spending in one field. Balanced spending is worse IMHO.
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Unbalanced spending may be more effective in the long run, but it always has costs. You may be able to outpace the loss by increasing your income faster using other methods (increasing economy bonuses from social improvements or starbases, improving economy or trade abilities from research, opening trade routes faster, etc.), but the closer to balanced your spending is, the more income you'll earn during a single turn's time.
~SDC~