Hi Ishitaa,
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.
Hope this helps?
Regards
Nidhi