To understand the difference between budget and ad spend, let's take a look at the mechanics of programmatic marketing.
Programmatic advertising uses a process called real-time bidding (RTB) to determine ad placement.
Real-time bidding: the buying and selling of online ad impressions through instantaneous auctions that occur in the time it takes a webpage to load.
When a person opens a website, the website signals that it has ad space available, and an ad exchange opens to accept bids. Advertisers who win the bid get to show their ads. Advertisers who consistently lose won't spend their budget and their ads will remain unseen.
Budgets indicate how much the advertiser is willing to spend.
Ad spend is how much the advertiser spends on ads after placing bids.
Successful campaigns have an ad spend that equals their budget. If you have a hard time reaching your budget, consider increasing your bid amounts or targeting a larger audience.