Background: Modern software uses many third-party libraries and frameworks as dependencies. Known vulnerabilities in these dependencies are a potential security risk. Software composition analysis (SCA) tools, therefore, are being increasingly adopted by practitioners to keep track of vulnerable dependencies. Aim: The goal of this study is to understand the difference in vulnerability reporting by various SCA tools. Understanding if and how existing SCA tools differ in their analysis may help security practitioners to choose the right tooling and identify future research needs. Method: We present an in-depth case study by comparing the analysis reports of 9 industry-leading SCA tools on a large web application, OpenMRS, composed of Maven (Java) and npm (JavaScript) projects. Results: We find that the tools vary in their vulnerability reporting. The count of reported vulnerable dependencies ranges from 17 to 332 for Maven and from 32 to 239 for npm projects across the studied tools. Similarly, the count of unique known vulnerabilities reported by the tools ranges from 36 to 313 for Maven and from 45 to 234 for npm projects. Our manual analysis of the tools' results suggest that accuracy of the vulnerability database is a key differentiator for SCA tools. Conclusion: We recommend that practitioners should not rely on any single tool at the present, as that can result in missing known vulnerabilities. We point out two research directions in the SCA space: i) establishing frameworks and metrics to identify false positives for dependency vulnerabilities; and ii) building automation technologies for continuous monitoring of vulnerability data from open source package ecosystems.