Why does powershell loop more times than I expect?

Question

I'm trying to change the names of 561 images using powershell. The first and second changes met my expectations, but when I tried to undo the second change a third time, the results did not meet my expectations.

here is my code:

PS E:\PART_5> $a = 1
PS E:\PART_5> Get-ChildItem | ForEach-Object {
>> $name = "newName_$a" + $_.Extension
>> Rename-Item -Path $_.FullName -NewName $name
>> $a = $a + 1
>> }

PS E:\PART_5> $a = 1
PS E:\PART_5> Get-ChildItem | ForEach-Object {
>> $name = "newestName_$a" + $_.Extension
>> Rename-Item -Path $_.FullName -NewName $name
>> $a = $a + 1
>> }

PS E:\PART_5> $a = 1
PS E:\PART_5> Get-ChildItem | ForEach-Object {
>> $name = "newName_$a" + $_.Extension
>> Rename-Item -Path $_.FullName -NewName $name
>> $a = $a + 1
>> }

and results:

First execution result	Second execution result	Third execution result
newName_1.jpg	newestName_1.jpg	newName_1000.jpg
newName_2.jpg	newestName_2.jpg	newName_1001.jpg
...	...	...
newName_561.jpg	newestName_561.jpg	newName_1560.jpg

To test I output the variable "a" in the process and the most amazing thing happened.

PS E:\PART_5> $a = 1
PS E:\PART_5> Get-ChildItem | ForEach-Object {
>> $name = "newestName_$a" + $_.Extension
>> Rename-Item -Path $_.FullName -NewName $name
>> $a
>> $a = $a + 1
>> }

PS E:\PART_5> $a = 1
PS E:\PART_5> Get-ChildItem | ForEach-Object {
>> $name = "newName_$a" + $_.Extension
>> Rename-Item -Path $_.FullName -NewName $name
>> $a
>> $a = $a + 1
>> }

When I loop through these two pieces of code, I find that "newestName" is always correct, looping "a" from 1 to 561; and "newName" is always wrong, and looping from 1 to 1560.

I don't know what the problem is and I hope someone can tell me.