Storage variable clashing

a.cairo

%lang starknet

from starkware.cairo.common.cairo_builtins import HashBuiltin

@storage_var
func balance() -> (res : felt):
end

func a_increase_balance{
        syscall_ptr : felt*, pedersen_ptr : HashBuiltin*,
        range_check_ptr}(amount : felt):
    let (res) = balance.read()
    balance.write(res + amount)
    return ()
end

func a_get_balance{
        syscall_ptr : felt*, pedersen_ptr : HashBuiltin*,
        range_check_ptr}() -> (res : felt):
    let (res) = balance.read()
    return (res)
end

b.cairo

%lang starknet

from starkware.cairo.common.cairo_builtins import HashBuiltin

@storage_var
func balance() -> (res : felt):
end

func b_increase_balance{
        syscall_ptr : felt*, pedersen_ptr : HashBuiltin*,
        range_check_ptr}(amount : felt):
    let (res) = balance.read()
    balance.write(res + amount)
    return ()
end

func b_get_balance{
        syscall_ptr : felt*, pedersen_ptr : HashBuiltin*,
        range_check_ptr}() -> (res : felt):
    let (res) = balance.read()
    return (res)
end

c.cairo

%lang starknet

from starkware.cairo.common.cairo_builtins import HashBuiltin

from openzeppelin.a import a_get_balance, a_increase_balance
from openzeppelin.b import b_get_balance, b_increase_balance

@external
func increase_balance_a{
        syscall_ptr : felt*, pedersen_ptr : HashBuiltin*,
        range_check_ptr}(amount : felt):
    a_increase_balance(amount)
    return ()
end

@external
func increase_balance_b{
        syscall_ptr : felt*, pedersen_ptr : HashBuiltin*,
        range_check_ptr}(amount : felt):
    b_increase_balance(amount)
    return ()
end

@view
func get_balance_a{
        syscall_ptr : felt*, pedersen_ptr : HashBuiltin*,
        range_check_ptr}() -> (res : felt):
    let (res) = a_get_balance()
    return (res)
end

@view
func get_balance_b{
        syscall_ptr : felt*, pedersen_ptr : HashBuiltin*,
        range_check_ptr}() -> (res : felt):
    let (res) = b_get_balance()
    return (res)
end

test_clash.py

import pytest
from starkware.starknet.testing.starknet import Starknet
from utils import contract_path


@pytest.mark.asyncio
async def test_clash():
    starknet = await Starknet.empty()
    public_contract = await starknet.deploy(
        contract_path("openzeppelin/c.cairo")
    )

    await public_contract.increase_balance_a(99).invoke()
    await public_contract.increase_balance_b(100).invoke()

    balance_one = await public_contract.get_balance_a().call()
    print("Should be 99: ", balance_one.result.res)  # prints 199

    balance_two = await public_contract.get_balance_b().call()
    print("Should be 100: ", balance_two.result.res). # prints 199