in American football, why is a "sack record" of a player is rated in a decimal form?

When a QB gets sacked, it counts as 1 sack.

However, there are situations where 2 defensive players may sack the QB at the same time or one defensive player may start to bring him down and another brings him all the way down.

In these situations, where there is a tackle with an assist, each defensive player may be credited with a 1/2 (or 0.5) of a sack.

They rarely can do a sack all by themselves. So in the example that you used they either helped a sack or got help do bring down the QB.

Sacks and tackles are recorded with a decimal because sometimes more than on person is involved in the play. In this case, multiple players are credited with the resulting play

Well they rarely can do a sack all by themselves. So in your example the guy either got help on a sack or they helped someone else on a sack.

solo sacks are recorded as whole numbers and non-solo sacks... when more than one person is involved in the sack it gets recorded as a half sack... or .5

When someone sacks someone by themselves its equal to=1.0

When two people sack the quarterback its equal to=.5

HOPE IT HELPS