Allocation memory change from 0 to some bytes at 4783 iteration

[allevo]

Hi!

I've a memory leak in an application. After some search I found this crate and restrict the area of my problem.
After some tries I have a strange behavior:

use jemalloc_ctl::{stats, epoch};
use jemallocator;
#[global_allocator]
static ALLOC: jemallocator::Jemalloc = jemallocator::Jemalloc;

#[actix_rt::main]
async fn main() -> std::io::Result<()> {
    let my_structs = vec![3];

    let allocated_bytes: usize = current();
    {
        for _i in 0..4782 {
            let _a: Vec<&i32> = my_structs
                .iter()
                .collect();
        }
    } // here dealloc all variables

    let allocated_bytes_after: usize = current();
    println!("before allocated_bytes: {}", allocated_bytes);
    println!("after   allocated_bytes: {}", allocated_bytes_after);
    println!("DIFF allocated_bytes: {}", allocated_bytes_after - allocated_bytes);

    Ok(())
}

fn current() -> usize {
    epoch::advance().unwrap();
    stats::allocated::read().unwrap()
}

The code above runs correctly printing 0 bytes as diff. On my PC the output is:

before allocated_bytes: 244960
after  allocated_bytes: 244960
DIFF allocated_bytes: 0

Anyway changing the magic number from 4782 to 4783, the program exits with panic because the difference is less than 0.

before allocated_bytes: 244960
after  allocated_bytes: 244376
thread 'main' panicked at 'attempt to subtract with overflow', src/main.rs:23:42
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Panic in Arbiter thread.

I don't know if this problem is linked to my origina one but this behavior is very strange and I would like to report it here.

Why the allocation change from 4782 to 4783? Why 4783?

I'm using:

$ cargo version
cargo 1.43.0 (2cbe9048e 2020-05-03)