BST Tree, Search function is giving odd times (measuring of search function) - rust

Question

I am trying to measure the time my BST search func takes to search an element in a 100-10,000 elements BST, with sorted & unsorted values.
Link to playground:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=d6708d951f80d58cdb77bc5fac1fc514
but I am getting weird results (in nanoseconds)
for 100 items:
BST - sorted: 33ns
BST - unsorted: 46ns
for 1,000 items:
BST - sorted: 10ns
BST - unsorted: 6ns
Seems impossible that the array is 10x times bigger and the time it took was way faster.
BST Search function:
pub fn search(&mut self, val: i32) -> bool {
if val == self.value {
true
} else {
if val <= self.value {
match self.left {
None => false,
Some(ref mut node) => node.search(val),
}
} else {
match self.right {
None => false,
Some(ref mut node) => node.search(val),
}
}
}
}
BST Insert function:
pub fn insert(&mut self, val: i32) {
if val <= self.value {
match self.left {
None => {
self.left = Some(Box::new(BSTNode {
value: val,
left: None,
right: None,
}))
}
Some(ref mut node) => node.insert(val),
}
} else {
match self.right {
None => {
self.right = Some(Box::new(BSTNode {
value: val,
left: None,
right: None,
}))
}
Some(ref mut node) => node.insert(val),
}
}
}
Timing function:
pub fn bst_search_timing(original_arr: &mut Vec<i32>) -> u128 {
let copy_arr: Vec<i32> = original_arr.to_vec();
let mut bst = BSTNode::new(1);
for item in copy_arr {
bst.insert(item);
}
let now = Instant::now();
bst.search(rand::random::<i32>());
let elapsed = now.elapsed();
return elapsed.as_nanos();
}
Main.rs:
let mut step = 100;
let times = 100;
let max = 10000;
while step <= max {
let mut bst_search_sum = 0;
let mut bst_search_unsorted_sum = 0;
let mut sorted_arr: Vec<i32> = vec![];
for x in 0..step {
sorted_arr.push(x);
}
for _ in 0..times {
let mut unsorted_arr: Vec<i32> = sorted_arr.to_vec();
unsorted_arr.shuffle(&mut thread_rng());
bst_search_sum += timing::bst_search_timing(&mut sorted_arr);
bst_search_unsorted_sum += timing::avl_search_timing(&mut sorted_arr);
}
And then I print the average:
println!(
"Average for {} elements: {}ns - BST_SEARCH: SORTED ARR",
step,
bst_search_sum / times
);
println!(
"Average for {} elements: {}ns - BST_SEARCH: UNSORTED ARR",
step,
bst_search_unsorted_sum / times
);