I know that different systems will track their analytics differently. However when comparing my conversion stats from Google Ads to Shopify's Analytics the discrepancy in conversions were around 25-30% off. (i.e. Google says there were 130 conversions due to marketing and Shopify says only 100). This applies to both the conversion amount and conversion value.
That was over a three month time span of Oct 2021 - Dec 2021 with over 5,000+ transactions so I think that is enough to be statistically significant.
You are totally right. 30% sounds more than it should be. We mostly observe 5% to 15% with the Analyzify clients. Is your store based in Europe and are you using any cookie consent apps? There might be something that is blocking your scripts. I can try helping more if you provide more information here.