Sage 200 Evolution Version 10.1.3.11 Voucher Management : Voucher Reference Numbers Skipped yet the Client is using Automatic Numbering.

Hi Colleagues,

Why would the voucher reference numbers be skipped yet tehy are set to automatic numbering?

A client is using Automating Reference Numbering in Voucher Management but After processing the GL Vouchers, some Numbers are being skipped, for example from 19 to 21 and the system skips 20. As seen in the Screen shot attached, directed by an arrow.

Thank you