There might be two phenomena going on. One is that individual perceptions are relative ... as senses like taste or smell can differ from one person to another. What one person prefers or senses as tasting sweet can be different than what you sense as sweet.
How does one person accurately describe the taste of something to another person? Wine lovers often read wine writers who wax on with absurd descriptions (using adjective like flinty, grassy...etc.) of taste experiences ... loaded with vagueness. But to relate the experience you'll have is nearly impossible. There's really no way to be sure two people have the same experience with their senses. So your sister might taste the exact same strawberry as you as being sweet but you sense or PREFER something a bit different.
The other is the variance in strawberries themselves. Less ripe strawberries are a bit tart and ripe ones are sweeter. But your preference or aversion to any tartness might be skewing your reaction to the taste.