Yes, Ecwid rounds weight values up to two decimals. As far as I understand, your products are typically under 1kg, so, I think, you should use grams as a weight unit in your store. It would be more convenient. If you decide to do so, you will need to change your shipping weight limits accordingly (0 - 249g, 250-499g). You can change the weight unit in your control panel
(System settings -> General -> Formats & Units).
Please let me know if you need any further assistance.