Its pretty normal... we tend to rush through a swatch, and slow down to knit.. and this change in tempo changes the gauge.
You can knit a sleeve (or both sleeves as a swatch.. a slightly too loose sleeve can be alterered by leaving out an increase or two... a slightly tight one, buy adding..
a sleeve lets you start knitting and lets you use your knitting as a swatch!
you can also slow down and relax and enjoy knitting your swatch.. to value it, (and not rush through it!) it will likely be closer to your normal knitting then!