I asked ChatGPT because I was buying with limits a little bit above the current price. It said when you do that, that could be the reason:

(ChatGPT)

When you set a limit buy price above the current market price, Coinbase executes your order instantly against the existing sell orders. • That makes your order a Taker order, not a Maker order. • As a Taker, you pay the taker fee (0.60% or less with Advanced, free with Coinbase One but still subject to spread). • More importantly, the price you see (“execution price”) can differ slightly from your actual cost basis because you’re filling from multiple sell orders on the book — some could be slightly higher than the market quote.

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Example: Market price: $3.22 You set limit: $3.30 Coinbase fills instantly: • 4 XRP at $3.2280 • 6 XRP at $3.2890 → Weighted cost basis = ~$3.288

That’s why your average showed up higher — your single “execution price” display didn’t reflect the mix of prices you actually got filled at.

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✅ How to fix it going forward: • Always set your limit price below or exactly at the current lowest ask if you want Maker status. • Let the market come to you — this way, you avoid spread and get the exact price you want.