The attribution method may be different. One platform may have a conversion window of 30 days while another one may have a conversion window of 90 days.

Some platforms have last-click attribution while others use first-click.

Another service reporting conversions may be combining organic and paid conversions. Your platform's dashboard will only show paid conversions.

Ad blockers block almost every conversion tag from firing. A portion of your visitors are probably using ad blockers which can skew the conversion count.