Shop Not Calculating Totals Correctly (extra 1p)

Hi

I have a shop selling items priced at £2.90 - they're in the shop as products with a £2.90 price (including tax).

In the cart they are totalled up in lines correctly (so 4 items cost £11.60 - this is correct).

I have 12 items in the cart, the total should be £34.80 but is showing as £34.81.

Any ideas why this might be? I have a single tax entry at 20 for UK - everything else is absolutely standard.

Thanks, Jack

Diagnostic incase it's helpful:

    Perch Runway: 2.8.32, PHP: 5.6.27-0+deb8u1, MySQL: 5.5.53, with PDO
    Server OS: Linux, apache2handler
    Installed apps: content (2.8.32), assets (2.8.32), categories (2.8.32), perch_blog (5.0), perch_events (1.9.3), perch_forms (1.8.3), perch_shop_orders (1.0.10), perch_shop_products (1.0.10), perch_shop (1.0.10), perch_members (1.5)
    App runtimes: <?php $apps_list = array( 'content', 'categories', 'perch_blog', 'perch_members', 'perch_shop', 'perch_events', 'perch_forms', );
    PERCH_LOGINPATH: /perch
    PERCH_PATH: /srv/whitby-brewery.com/public/htdocs/perch
    PERCH_CORE: /srv/whitby-brewery.com/public/htdocs/perch/core
    PERCH_RESFILEPATH: /srv/whitby-brewery.com/public/htdocs/perch/resources
    Image manipulation: GD Imagick
    PHP limits: Max upload 30M, Max POST 30M, Memory: 128M, Total max file upload: 30M
    F1: 2edba60ed1f613d6dd804feb202456a2
    Resource folder writeable: Yes
    HTTP_HOST: www.whitby-brewery.com
    DOCUMENT_ROOT: /srv/whitby-brewery.com/public/htdocs/
    REQUEST_URI: /perch/core/settings/diagnostics/
    SCRIPT_NAME: /perch/core/settings/diagnostics/index.php