发送域名/IP未在SPF记录中，为何Gmail仍判定SPF通过？

我查阅了相关内容，但均围绕SPF不通过的情况展开。我遇到了反常情况：本应在Gmail中判定SPF失败，但实际却通过了。

我使用电商平台Sharetribe，该平台通过SendGrid（Twilio）发送客户沟通邮件。我已为SendGrid配置3条CNAME记录，其中2条为DKIM密钥相关记录，另一条如下：

12345678.u1234.sendgrid.net
flx.domainkey.12345678.u1234.sendgrid.net
flx2.domainkey.12345678.u1234.sendgrid.net

但尚未为该第三方平台配置SPF记录（平台未提供相关支持），因此我对SPF判定产生疑问：是否相关标准或协议已更新？

我的SPF记录示例如下：
"v=spf1 include:our.email.service ~all"
该记录未包含Sharetribe或SendGrid。

那么为何Gmail会判定SPF通过？

以下是客户收到的Gmail邮件头示例：

ARC-Authentication-Results: i=1; mx.google.com;
   dkim=pass header.i=@our.domain header.s=flx header.b="dca/fgbb";
   dkim=pass header.i=@sendgrid.info header.s=smtpapi header.b=dfgdfg;
   spf=pass (google.com: domain of bounces+34564567-456- recipient=gmail.com@em404.our.domain designates 168.245.24.68 as permitted sender) smtp.mailfrom="bounces+34564567-456-recipient=gmail.com@em404.our.domain"
Return-Path: <bounces+34564567-456-recipient=gmail.com@em404.our.domain>
Received: from o1.flex-mail.sharetribe.com (o1.flex-mail.sharetribe.com. [168.245.24.68])
    by mx.google.com with ESMTPS id bx41-sdgfhbsrgberthberhbreybtyrbtyjbtyjbtyjb.111.2023.10.31.15.06.27
    for <recipient@gmail.com>
    (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128);
    Tue, 31 Oct 2023 15:06:28 -0700 (PDT)
Received-SPF: pass (google.com: domain of bounces+34564567-456-recipient=gmail.com@em404.our.domain designates 168.245.24.68 as permitted sender) client-ip=168.245.24.68;
Authentication-Results: mx.google.com;
   dkim=pass header.i=@our.domain header.s=flx header.b="dca/fgbb";
   dkim=pass header.i=@sendgrid.info header.s=smtpapi header.b=dfgdfg;
   spf=pass (google.com: domain of bounces+34564567-456-recipient=gmail.com@em404.our.domain designates 168.245.24.68 as permitted sender) smtp.mailfrom="bounces+34564567-456-recipient=gmail.com@em404.our.domain"

内容的提问来源于stack exchange，提问作者TheArchitecta