Slippage is asymmetric when a broker's fills move against clients more often, or by more, than they move in clients' favour. Price genuinely changes between the moment an order is sent and the moment it is filled, so any live execution model produces slippage in both directions; a persistently one-sided pattern is what suggests the difference is being kept rather than passed on.
It is a question about a distribution, not about one bad fill, which is why best-execution obligations look at fills as a whole and why some brokers publish the share of orders filled at, better than, and worse than the requested price. You can examine your own account the same way: record requested and filled prices over a run of orders and compare the two directions, keeping news trades separate from ordinary conditions, since real volatility widens both tails at once.
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Guide
Order Types and Slippage Explained
Market, limit and stop orders trade price certainty against execution certainty — plus what slippage is, where it comes from, and what a stop-loss caps.