What was expected : 

/summon command summon the entity at correct coordinate 

What happened instead :

/summon command with coordinate

X: 100000.333333 will summon the entity at coordinate

X: 99999.666666

and /summon command with coordinate 

X: 100000.666666 will summon the entity at coordinate

X: 100000.333333

 

Each command used in the screenshot :

X: 100000

/summon minecraft:falling_block 100000 6 100031 {Time:1,NoGravity:1,Tags:["permanent","sprucefence1"],BlockState:{Name:spruce_fence,Properties:

{east:"false",north:"false",south:"false",west:"false",waterlogged:"false"}

}}

X: 100000.333333

/summon minecraft:falling_block 100000.333333 6 100031 {Time:1,NoGravity:1,Tags:["permanent","sprucefence1"],BlockState:{Name:spruce_fence,Properties:

{east:"false",north:"false",south:"false",west:"false",waterlogged:"false"}

}} 

X: 100000.666666

/summon minecraft:falling_block 100000.666666 6 100031 {Time:1,NoGravity:1,Tags:["permanent","sprucefence1"],BlockState:{Name:spruce_fence,Properties:

{east:"false",north:"false",south:"false",west:"false",waterlogged:"false"}

}}