In addition, if auto adjust campaign budget limits are defined in the portfolio - it allows AMO to play around with spend levels for campaigns and adsets(in case of facebook) to maintain spending for entire portfolio.
Hence, some of your campaigns/ad sets might spend more or less than the campaign/ad set budgets.
I assume your question is in relation to AMO? Actually budget in AMO is managed at Portfolio level. As per the Portfolio's objective, market demand, campaign performance, historical click and conversion data, AMO's technology bid for keywords in a campaign to enhance the conversions and utilize budget accordingly. Therefore, you might see some variation of spend between campaigns but in limit set at Portfolio level.